Intersystem base station handover

ABSTRACT

A mobile wireless telecommunications system includes base stations of a first type operating according to a first air interface, and base stations of a second type operating according to a second air interface. Methods and apparatus are provided for handing over a mobile station in the system from a first base station, which is of the first type, to a second base station, which is of the second type. A communications link is established over the first air interface between the mobile station and the first base station. Data are received from the mobile station responsive to a signal received by the mobile station over the second air interface from the second base station, substantially without breaking the communications link with the first base station. The mobile station is handed over from the first to the second base station responsive to the data received therefrom. In particular, a method of conducting intersystem handover from a multicarrier system to a direct spread system is provided. Timing synchronization is also advantageously made available through the mobile station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. applicationSer. No. 09/365,967, filed Aug. 2, 1999; which is a continuation-in-partof U.S. application Ser. No. 09/119,717, filed Jul. 20, 1998, bothassigned to the assignee of the present invention.

FIELD OF THE INVENTION

The present invention relates generally to wireless telecommunications,and specifically to advanced cellular telephone networks.

BACKGROUND OF THE INVENTION

The Global System for Mobile (GSM) telecommunications is used incellular telephone networks in many countries around the world. GSMoffers a useful range of network services and standards. Existing GSMnetworks are based on time-division multiple access (TDMA) digitalcommunications technology. In a TDMA-based cellular network, each mobilesubscriber unit communicates with only a single base station at anygiven time. When a subscriber moves from one cell to another, a “hardhandover” takes place, in which the base station with which thesubscriber has been communicating breaks off its link with thesubscriber, and a new base station takes over.

Code-division multiple access (CDMA) is an improved digitalcommunications technology, which affords more efficient use of radiobandwidth than TDMA, as well as a more reliable, fade-free link betweencellular telephone subscribers and base stations. The leading CDMAstandard is IS-95, promulgated by the Telecommunications IndustryAssociation (TIA).

This standard provides “soft handover” (or “handoff”) capability,wherein in moving from one cell to another, the subscriber unit istemporarily in contact with two or more base stations at the same time.This soft handover, which is made possible by the code-divisionapproach, decreases the likelihood of a loss of connection, which canhappen frequently in hard handovers.

PCT patent application PCT/US96/20764, which is incorporated herein byreference, describes a wireless telecommunications system that uses aCDMA air interface (i.e., basic RF communications protocols) toimplement GSM network services and protocols. Using this system, atleast some of the TDMA base stations (BSSs) and subscriber units of anexisting GSM network would be replaced or supplemented by correspondingCDMA equipment. CDMA BSSs in this system are adapted to communicate withGSM mobile switching centers (MSCs) via a standard GSM A-interface. Thecore of GSM network services is thus maintained, and the changeover fromTDMA to CDMA is transparent to users.

Hybrid cellular communications networks, incorporating both GSM and CDMAelements, are also described in PCT patent publications WO 95/24771 andWO 96/21999, and in an article by Tscha, et al., entitled “A SubscriberSignaling Gateway between CDMA Mobile Station and GSM MobileSwitching—Center,” in Proceedings of the 2nd International Conference onUniversal Personal Communications, Ottawa (1993), pp. 181-185, which areincorporated herein by reference. None of these publications deals withspecific issues of how to perform efficient handovers of subscriberunits between different base stations in such hybrid networks.

PCT patent application PCT/US97/00926, which is also incorporated hereinby reference, describes methods of intersystem handover between CDMA andTDMA BSSs in a hybrid GSM/CDMA telecommunications system. A GSM/TDMA BSSgenerates pilot beacon signals in accordance with CDMA technology.During a telephone call, a subscriber unit detects the pilot signals andnotifies a base station controller that the signals have been detected.The subscriber unit is then handed over from the CDMA to the TDMA BSSwithout interrupting the call.

The International Telecommunications Union recently requested thesubmission of proposed methods for providing high-rate data andhigh-quality speech services over wireless communication channels. Afirst of these proposals was issued by the Telecommunications IndustryAssociation, entitled “The cdma2000 ITU-R RTT Candidate Submission,” andhereinafter referred to as cdma2000. A second of these proposals wasissued by the European Telecommunications Standards Institute (ETSI),entitled “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTTCandidate Submission,” also known as “wideband CDMA,” and hereinafterreferred to as W-CDMA. A third proposal was submitted by U.S. TG 8/1,entitled “The UWC-136 Candidate Submission,” hereinafter referred to asEDGE. The contents of these submissions are public record and are wellknown in the art.

Two known radio-frequency (RF) interfaces for such so-called “thirdgeneration” wireless communication systems are the multicarrier (MC)over-the-air interface and the direct spread (DS) over-the-airinterface. A third generation system employing the MC air interface maybe a system that uses network signaling protocols specified by AmericanNational Standards Institute (ANSI) 41. The proposed cdma2000 system issuch a system. In the alternative, a system using an MC air interfacemay employ network signaling protocols defined by the Mobile ApplicationPart (MAP) of the GSM-MAP standard described above. Likewise, a systemmay employ a DS air interface and ANSI 41 network signaling protocols,or a DS air interface and MAP network signaling protocols. The proposedWCDMA system uses the DS air interface and MAP network signaling.

Just as for GSM and CDMA systems, intersystem handover is also necessaryin areas where MC system coverage (e.g., cdma2000 base stations) givesway to DS system coverage (e.g., WCDMA base stations), or vice versa. Itis also necessary to provide efficient timing synchronization betweenbase stations of the two systems in that are communicating with a mobileunit during the handover.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide methods andapparatus for use in a mixed TDMA/CDMA cellular communications network.

It is a further object of some aspects of the present invention toprovide improved methods and apparatus enabling handover of a subscriberunit between TDMA and CDMA base stations without interruptingcommunications.

In preferred embodiments of the present invention, a mixed GSM/CDMAcellular communications system includes both TDMA and CDMA basestations, jointly controlled by a mobile switching center (MSC). Systemsof this type are described generally in the above-mentioned PCT patentapplications, which are incorporated herein by reference. A subscriberunit in the system, also referred to herein as a mobile station (MS), iscapable of communicating with both types of base stations, byappropriately switching between TDMA and CDMA air interfaces, whilepreferably using GSM network protocols over both types of interface. Itis a feature of preferred embodiments of the present invention that thecommunications system may be based on an existing GSM/TDMAinfrastructure, with the addition of CDMA BSSs, and with substantiallyno other modification to the existing infrastructure.

In order to determine when a handover should take place, a MS incommunication with a current base station of one type (CDMA or TDMA)monitors RF signals originating from another base station, which may bea base station of the other type (TDMA or CDMA, respectively). A messagesequence between the current base station and the MS enables the MS toacquire appropriate synchronization information with regard to the newbase station, and report back on this information to the current basestation. The information is used by the system to enable the MS toestablish an air interface with the new base station, whereupon thehandover takes place without substantially interrupting communicationsbetween the MS and the network.

In the context of the present patent application, such handovers betweenbase stations are referred to as “mobile-assisted handovers.”Mobile-assisted handover is used in GSM and in CDMA systems known in theart, wherein a mobile station measures and reports on the strength ofsignals received from a base station transceiver in a neighboring cellbefore being handed over to that cell. In hybrid GSM/CDMA systems thathave been proposed to date, however, mobile stations are presumed to becapable of receiving signals from either a CDMA or a TDMA base stationat any given time (or a CDMA beacon associated with a TDMA base station,as in the above-mentioned PCT patent application PCT/US97/00926), butnot both, and are therefore not capable of providing this type ofassistance. The provision of mobile-assistance in accordance with theprinciples of the present invention enables handovers to be conductedmore smoothly and reliably than would otherwise be possible.

In some preferred embodiments of the present invention, the MS switchesbetween TDMA and CDMA operation in the course of a telephone call,according to instructions received from the base station with which theunit is in communication. Before the handover is to take place, the MSreceives signals from both TDMA and CDMA base stations, and reports backto the base station regarding the signals it is receiving. Theinformation thus reported is reported back to and used by the BSC toinitiate the handover. Preferably, the MS comprises a single radiotransceiver, and therefore, at any given moment the MS can communicatewith either the TDMA or CDMA base station, but not both. (In accordancewith the principles of IS-95, however, as described hereinabove, theunit can communicate with more than one CDMA base station at once.) Itis noted further that each GSM/TDMA base station has its ownsynchronization clock, to which the MSs in communication therewith aresynchronized, while the CDMA base stations are mutually synchronized toa real time of day. Therefore, in switching between the TDMA and CDMAstations, the MS in each case acquires and synchronizes its operation tothe appropriate clock signal without substantially interrupting thetelephone call.

In some of these preferred embodiments, the MS is in communication witha CDMA base station, when it is determined that the unit may be handedover to a GSM/TDMA base station. CDMA transmission by the MS transceiveris interrupted temporarily, during which time the unit performs a GSMneighbor scan, generally in accordance with GSM standards, to acquireand synchronize to the TDMA base station. Preferably, the CDMAtransmission is interrupted for a single frame, typically 20 ms long,creating an idle time slot in accordance with the IS95 standard. Afterthe TDMA base station is identified, and suitable messages have beenexchanged, a traffic channel between the base station is opened, and theMS is switched to the TDMA base station while interruption of atelephone call being conducted by the MS is substantially minimized.

In others of these preferred embodiments, the MS is in communicationwith a TDMA base station, when it is determined that the unit may behanded over to a CDMA base station. In order to synchronize with theCDMA station, the MS acquires the time of day, preferably by receivingan accurate time of day from the TDMA base station, wherein the GSMnetwork is provided with equipment necessary to generate and broadcastthe time of day. Preferably, the network includes a cell broadcastsystem (CBS), in accordance with the GSM standard, which is used toreceive the time of day, provided, for example, by the GlobalPositioning System (GPS) or received from one or more of the CDMA basestations, and broadcast it through the network to the MSs.Alternatively, the MS temporarily interrupts TDMA reception in order toacquire and synchronize to the time of day of the CDMA station. Thus,although a certain degradation of the signal may result from the TDMAtime slot(s) lost in this fashion, the mobile-assisted handover fromTDMA to CDMA is generally more reliable and less disturbing to a user ofthe MS than would otherwise be possible.

Although preferred embodiments are described herein with reference toMSs having a single transceiver for TDMA and CDMA use, it will beappreciated that the principles of the present invention may similarlybe applied using subscriber units and system hardware of other types,and particularly using a subscriber unit having separate or onlypartially integrated TDMA and CDMA transceivers.

There is therefore provided, in accordance with a preferred embodimentof the present invention, in a mobile wireless telecommunicationssystem, which includes base stations of a first type operating accordingto a first air interface, and base stations of a second type operatingaccording to a second air interface, a method for handing over a mobilestation in the system from a first base station, which is of the firsttype, to a second base station, which is of the second type, including:

establishing a communications link over the first air interface betweenthe mobile station and the first base station;

receiving data from the mobile station responsive to a signal receivedby the mobile station over the second air interface from the second basestation, substantially without breaking the communications link with thefirst base station; and

handing over the mobile station from the first to the second basestation responsive to the data received therefrom.

Preferably, receiving the data includes receiving a measurement ofsignal strength, and handing over the mobile station includes comparingmeasurements of signal strengths from the first and second base stationsand handing over the mobile station responsive to the comparison.Preferably, receiving the data includes applying a weighting factor tothe measurement of signal strength, wherein applying the weightingfactor includes varying the factor according to a network condition inthe system. Further preferably, applying the weighting factor includestransmitting a weighting factor over the communications link to themobile station, which applies the weighting factor to the measurement.

Preferably, receiving the data includes receiving an identification ofthe second base station based on decoding by the mobile station of thesignal received over the second air interface.

In a preferred embodiment, transmitting from the first base station tothe mobile station a list of frequencies of base stations of the secondtype in the system, such that the mobile station seeks to receive thesignal at a frequency in the list.

Preferably, handing over the mobile station includes transmitting ahandover command from the first base station. In a preferred embodiment,handing over the mobile station includes sending an initial transmissionover the second air interface responsive to the handover command, andthe method includes reacquiring the communications link over the firstair interface if the initial transmission over the second air interfaceis not successfully received.

Preferably, transmitting the handover command includes transmitting acommand over the first air interface that encapsulates parametersrelating to the second air interface. Most preferably, transmitting thecommand includes transmitting a command in accordance with a GSMstandard that encapsulates parameters defined in accordance with anIS-95 standard, wherein the parameters encapsulated include an IS-95long code.

Preferably, establishing the communications link and receiving the dataresponsive to the signal include establishing the link and receiving thesignal at the mobile station using a single RF transceiver in the mobilestation.

In a preferred embodiment, one of the first and second air interfacesincludes a TDMA interface, and the other of the interfaces includes aCDMA interface, wherein the TDMA interface preferably includes a GSMinterface, and wherein the CDMA interface is configured to convey GSMnetwork messages. Preferably, the CDMA interface is based on an IS-95standard.

Preferably, establishing the communications link includes using a singleradio resource management protocol layer to manage the first airinterface, and wherein handing over the mobile station includes usingthe single radio resource management protocol layer to manage the secondair interface.

Further preferably, receiving the data from the mobile station includesdefining an area of overlap between a first region served by the firstair interface and a second region served by the second air interface,and triggering the mobile station to receive the data when the mobilestation is in the area of overlap.

In a preferred embodiment, the first air interface includes a CDMAinterface, and wherein the second air interface includes a GSM/TDMAinterface, and receiving data from the mobile station includes gatingthe mobile station to interrupt a CDMA communications link so as toreceive and decode a GSM/TDMA signal. Preferably, gating the mobilestation includes interrupting CDMA communications for the duration of anIS-95 frame, wherein receiving the data includes receiving anidentification of the second base station based on decoding of GSMfrequency correction and synchronization channels of the signal by themobile station.

In another preferred embodiment, the first air interface includes aGSM/TDMA interface, and the second air interface includes a CDMAinterface, and receiving the data from the mobile station includescontrolling the mobile station to interrupt the communications link soas to receive and decode a CDMA signal.

Preferably, receiving the data includes conveying time of dayinformation through the GSM/TDMA interface. Further preferably,conveying the time of day information includes broadcasting time of dayinformation through the system using a GSM cell broadcast service,wherein broadcasting the time of day information includes receiving atime of day and an associated GSM frame number from a transceiver incommunication with a base station of the first type in the system.Preferably, the mobile station decodes a sync channel of the CDMA signalso as to derive the time or day.

Alternatively or additionally, receiving the data includes conveying aGSM cell broadcast service message to the mobile station to initiate asearch by the mobile station for a signal from a base station of thesecond type. Preferably, conveying the GSM cell broadcast servicemessage to the mobile station includes conveying the message so as to bereceived by the mobile station while the mobile station is operating ina dedicated mode.

Preferably, receiving the data from the mobile station includesreceiving an identification of a CDMA pilot beam decoded by the mobilestation. Further preferably, the method includes mapping the second basestation as a GSM base station so as to control the handover.

Preferably, controlling the mobile station includes controlling themobile station to receive the CDMA signal during a first TDMA time slotand to decode the signal during a subsequent TDMA time slot whilecommunicating with the base station over the TDMA interface so as togenerate the data to be received by the base station.

There is further provided, in accordance with a preferred embodiment ofthe present invention, a method for conveying time of day information toa mobile station in a GSM wireless telecommunications system, including:

inputting the time of day information to the system; and

broadcasting the information to the mobile station over the system.

Preferably, the GSM wireless telecommunications system includes a cellbroadcast system, and broadcasting the time of day information includesbroadcasting the information over the cell broadcast system. Preferably,broadcasting the time of day information includes broadcasting a messageso as to be received by the mobile station while the station isoperating in a dedicated mode.

Further preferably, broadcasting the time of day information includesreceiving a time of day and an associated GSM frame number from atransceiver in communication with the system, and the method includessynchronizing the mobile station to a CDMA transmission signal using thetime of day information.

In a preferred embodiment, the method includes determining a location ofthe mobile station responsive to a transmission thereby of the time ofday information to a plurality of base stations in the system.

Preferably, inputting the time of day includes opening a data call froma transceiver having the time of day information to the cell broadcastcenter, wherein opening the data call preferably includes receiving timeof day information from a GPS device. Alternatively, opening the datacall includes receiving time of day information from a CDMA cellassociated with the GSM system.

There is further provided, in accordance with a preferred embodiment ofthe present invention, in a GSM mobile wireless telecommunicationssystem, which includes a first base station subsystem and a second basestation subsystem, at least one of which subsystems operates accordingto a CDMA air interface, a method for handing over a mobile station inthe system from first to the second base station subsystem, including:

mapping the at least one of the first and second subsystems thatoperates according to the CDMA air interface as a GSM/TDMA subsystem;

establishing a communications link between the mobile station and thefirst base station subsystem, so that the mobile station receives afirst signal from the first base station subsystem;

receiving data from the mobile station responsive to a second signalreceived by the mobile station from the second base station subsystem,substantially without breaking the communications link with the firstbase station subsystem;

comparing the strengths of the first and second signals, substantiallyas though both the first and second base station subsystems wereGSM/TDMA subsystems; and

handing over the mobile station from the first to the second basestation subsystem responsive to comparison of the signal strengths.

Preferably, mapping the at least one of the subsystems that operatesaccording to the CDMA air interface includes assigning to the subsystema GSM frequency and location.

Further preferably, establishing the communications link and handingover the mobile station include conveying messages between the first andsecond subsystems and a mobile switching center in the system via a GSMA-interface. Preferably, both the first and second base stationsubsystems operate according to the CDMA air interface, wherein handingover the mobile station includes conveying a new IS-95 long code throughthe A-interface, substantially without violating A-interface protocols.

Preferably, receiving the data from the mobile station includes applyinga weighting factor to the second signal, and wherein comparing thestrengths of the signals includes comparing the weighted signal, whereinapplying the weighting factor includes conveying the weighting factor tothe mobile station, which applies the weighting factor to the secondsignal. Preferably, applying the weighting factor includes varying thefactor according to a network condition in the system.

There is also provided, in accordance with a preferred embodiment of thepresent invention, wireless communications apparatus, for use in amobile telecommunications system, including:

a base station of a first type which transmits and receives a firstsignal according to a first air interface;

a base station of a second type which transmits and receives a secondsignal according to a second air interface; and

a mobile station, which receives the second signal over the second airinterface from the base station of the second type while maintaining acommunication link over the first air interface with the base station ofthe first type, and which transmits data to the base station of thefirst type responsive to the second signal so that the mobile station ishanded over from the first to the second base station responsive to thetransmitted data.

Preferably, the data transmitted by the mobile station includes ameasurement of signal strength, such that the mobile station is handedover responsive to a comparison of signal strengths of the first andsecond signals. Preferably, a weighting factor is applied to themeasurement of signal strength, wherein the weighting factor is variedaccording to a network condition in the system. Preferably, theweighting factor is transmitted over the communications link to themobile station, which applies the weighting factor to the measurement.

Further preferably, the mobile station decodes the second signal todetermine an identification of the base station of the second type.

Preferably, the base station of the first type transmits to the mobilestation a list of frequencies of mobile stations of the second type inthe system, such that the mobile station seeks to receive the secondsignal at a frequency in the list.

Preferably, the base station of the first type transmits a handovercommand to the mobile station, whereby the mobile station is handed overfrom the first to the second base station. In a preferred embodiment, aninitial transmission is sent over the second air interface responsive tothe handover command, and the mobile station reacquires thecommunications link over the first air interface if the initialtransmission over the second air interface is not successfully received.

Preferably, the handover command encapsulates parameters relating to thesecond air interface. Most preferably, the command in substantially inaccordance with a GSM standard and encapsulates parameters defined inaccordance with an IS-95 standard, wherein the parameters encapsulatedinclude an IS-95 long code.

Further preferably, the mobile station includes a single RF transceiverwhich communicates with both the base stations of the first and secondtypes.

In a preferred embodiment, one of the first and second air interfacesincludes a TDMA interface, and the other of the interfaces includes aCDMA interface, wherein the TDMA interface preferably includes a GSMinterface, and wherein the CDMA interface is configured to convey GSMnetwork messages. Preferably, the CDMA interface is based on an IS-95standard. Further preferably, the mobile station uses a single radioresource management protocol layer to manage both the first and secondair interfaces.

Preferably, the base station triggers the mobile station to receive thesecond signal over the second air interface when the mobile station isin an area of overlap between a first region served by the first airinterface and a second region served by the second air interface

In a preferred embodiment, the first air interface includes a CDMAinterface, and the second air interface includes a GSM/TDMA interface,and the base station of the first type gates the mobile station tointerrupt the communications link so as to receive and decode a GSMsignal.

Preferably, the mobile station interrupts the link for the duration ofan IS-95 frame.

Further preferably, the mobile station processes the second signal todecode GSM frequency correction and synchronization channels of thesignal.

In another preferred embodiment, the first air interface includes aGSM/TDMA interface, and the second air interface includes a CDMAinterface, and the base station of the first type controls the mobilestation to interrupt the communications link so as to receive and decodea CDMA signal.

Preferably, the base station of the first type conveys time of dayinformation to the mobile station through the GSM/TDMA interface.Preferably, the apparatus includes a GSM cell broadcast center, whichconveys the time of day information through the system to the mobilestation using a GSM cell broadcast service, wherein the cell broadcastcenter receives the time of day information and an associated GSM framenumber from a transceiver in communication with a base station of thefirst type in the system.

Alternative or additionally, the mobile station decodes asynchronization channel of the CDMA signal so as to derive the time ofday.

Preferably, the GSM cell broadcast center conveys a cell broadcastservice message to the mobile station to initiate a search by the mobilestation for the second signal, wherein the mobile station receives thecell broadcast service message while the mobile station is operating ina dedicated mode.

Alternatively or additionally, the mobile station processes the CDMAsignal to identify a CDMA pilot beam.

Preferably, the mobile station receives the CDMA signal during a firstTDMA time slot and processes the signal during a subsequent TDMA timeslot while communicating with the base station over the TDMA interfaceso as to generate the data for transmission to the base station.

There is further provided, in accordance with a preferred embodiment ofthe present invention, apparatus for conveying time of day informationto a mobile station in a GSM wireless telecommunications system,including a cell broadcast center, which broadcasts the information tothe mobile station using a GSM cell broadcast system.

Preferably, the apparatus includes a transceiver in communication withthe system, which transmits a time of day and an associated GSM framenumber to the cell broadcast center, wherein the transceiver opens adata call through the system to the cell broadcast center so as toconvey the time of day and the associated frame number thereto.

Preferably, the mobile station is synchronized to a CDMA transmissionsignal using the time of day information.

Further preferably, the mobile station receives the information from thecell broadcast system while operating in a dedicated mode.

There is moreover provided, in accordance with a preferred embodiment ofthe present invention, apparatus for inputting time of day informationto a communications controller in a wireless telecommunications system,including:

a clock signal receiver, which receives the time of day information froma clock source; and

a radio transceiver, which receives the time of day information from theclock signal receiver, and which opens a data call through the system tothe communications controller so as to convey the information thereto.

Preferably, the communications controller includes a GSM cell broadcastcenter, wherein the radio transceiver receives a GSM frame number from abase station in the system, and conveys the frame number to the cellbroadcast center together with the time of day information.

Preferably, the clock signal receiver includes a radio receiver whichreceives the time of day information from a CDMA communications cell,wherein the radio transceiver includes the radio receiver.

Alternatively, the clock signal receiver includes a GPS device.

There is additionally provided, in accordance with a preferredembodiment of the present invention, apparatus for mobile wirelesstelecommunications in a GSM telecommunications system, including:

a mobile station; and

first and second base station subsystems, transmitting first and secondsignals to the mobile station, at least one of which is a CDMA signal,and both of which subsystems are mapped in the GSM system as GSM basestation subsystems,

wherein the mobile station is handed over from the first to the secondsubsystem responsive to a comparison of the strengths of the first andsecond signals received by the mobile station, substantially as thoughboth the first and second base station subsystems operated according toa GSM/TDMA air interface.

Preferably, the subsystem transmitting the CDMA signal is assigned a GSMfrequency and location in the system. Further preferably, messages areconveyed between the first and second subsystems and a mobile switchingcenter in the system via a GSM A-interface, wherein both the first andsecond signals include CDMA signals. Preferably, a new IS-95 long codeis conveyed through the A-interface from the second to the firstsubsystem in order to hand over the mobile station, substantiallywithout violating A-interface protocols.

Preferably, the mobile station applies a weighting factor to the secondsignal before the signal strengths are compared.

There is further provided, in accordance with a preferred embodiment ofthe present invention, a mobile station for use in a wirelesstelecommunications system including CDMA and TDMA base stations,including:

a single mobile radio transceiver, which communicates with the CDMA andTDMA base stations; and

a modem unit, which encodes signals for transmission by the mobiletransceiver and decodes signals received thereby, such that the signalsare CDMA-encoded for communication with the CDMA base station andTDMA-encoded for communication with the TDMA base station.

Preferably, the modem unit encodes the signals in accordance with GSMradio interface layer protocols.

Further preferably, the mobile station receives and processes a signalfrom one of the CDMA and TDMA base stations substantially withoutbreaking a communications link existing between the mobile station andthe other one of the CDMA and TDMA base stations.

There is also provided, in accordance with a preferred embodiment of thepresent invention, a method for conveying messages to a plurality ofmobile stations operating in a dedicated mode in a GSM wirelesstelecommunications system including a cell broadcast service, including:

broadcasting the messages to the mobile stations over the cell broadcastservice; and

receiving the messages at the mobile stations substantially withoutterminating the dedicated mode operation of the mobile stations.

Preferably, broadcasting the messages includes sending time-of-dayinformation or, alternatively or additionally, broadcasting a searchtrigger message.

There is additionally provided, in accordance with a preferredembodiment of the present invention, apparatus for mobile wirelesstelecommunications in a GSM telecommunications system, including:

a cell broadcast center, which broadcasts messages over a cell broadcastsystem; and

a mobile station, which receives the messages while communicating in adedicated mode, substantially without terminating the dedicated modecommunications.

Preferably, the cell broadcast center broadcasts time-of-day informationor, alternatively or additionally, a search trigger message.

There is additionally provided, in accordance with a preferredembodiment of the present invention, a mobile station for use in awireless telecommunications system including CDMA and TDMA basestations, including:

at least one mobile radio transceiver, which communicates with the CDMAand TDMA base stations; and

a modem unit, which processes signals for transmission by the at leastone transceiver and received thereby in accordance with a communicationsprotocol stack, such that the signals are CDMA-encoded for communicationwith the CDMA base station and TDMA-encoded for communication with theTDMA base station, the stack including a single radio resourcemanagement protocol layer which controls the communications with boththe CDMA and TDMA base stations.

Preferably, the radio resource management protocol layer performssubstantially all of the functions of a GSM Radio Interface Layer 3 RRsublayer.

Further preferably, the radio resource management protocol layercontrols a handover of the mobile station from one of the base stationsto another of the base stations.

There is moreover provided, in accordance with a preferred embodiment ofthe present invention, in a GSM mobile wireless telecommunicationssystem, which includes base station subsystems at least some of whichoperate according to a CDMA air interface, a method for controllingcommunications of a mobile station in the system with the base stationsubsystems, including:

sending and receiving signals between the mobile station and one of thebase station subsystems over the CDMA air interface; and

controlling the sending and receiving using a radio resource managementcommunications protocol layer that performs substantially all of thefunctions of a GSM Radio Interface Layer 3 RR sublayer.

Preferably, the system further includes base station subsystems whichoperate according to a TDMA air interface and the method includes:

sending and receiving signals between the mobile station and one of thebase station subsystems over the TDMA air interface,

wherein controlling the sending and receiving includes using the singleradio resource management communications protocol layer to controlsending and receiving of signals over both the CDMA and TDMA airinterfaces.

Further preferably, the method includes handing over the mobile stationbetween TDMA and CDMA base stations, wherein the handover is controlledby the radio resource management communications protocol layer.

In one aspect of the invention, a method of facilitating intersystemhandover of communications between a mobile station and at least onebase station of a first wireless communications system to the mobilestation and at least one base station of a second wireless communicationsystem is provided. The method advantageously includes the steps oftransmitting a message from the mobile station to the at least one basestation of the first wireless communications system, the messageincluding timing information about the at least one base station of thesecond wireless system; and determining relative timing between the atleast one base station of the first wireless communications system andthe at least one base station of the second wireless communicationssystem.

In another aspect of the invention, a method of performing intersystemhandover of communications between a mobile station and at least onebase station of a first wireless communications system to the mobilestation and at least one base station of a second wireless communicationsystem is provided. The method advantageously includes the steps oftransmitting a message from the at least one base station of the firstwireless communications system to the mobile station, the messageincluding timing information about the at least one base station of thesecond wireless system; and using the transmitted timing information tofacilitate intersystem handover of communications between the mobilestation and the at least one base station of the first wirelesscommunications system to the mobile station and the at least one basestation of the second wireless communication system.

The present invention will be more fully understood from the followingdetailed description of the preferred embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a hybrid GSM/CDMA cellularcommunications system, in accordance with a preferred embodiment of thepresent invention;

FIG. 2A is a schematic block diagram illustrating communicationsprotocols between a mobile station and base station subsystems in thesystem of FIG. 1, in accordance with a preferred embodiment of thepresent invention;

FIG. 2B is a schematic block diagram of a hybrid GSM/CDMA mobilestation, in accordance with a preferred embodiment of the presentinvention;

FIGS. 3A and 3B are schematic block diagrams illustrating communicationsprotocol stacks between elements of the system of FIG. 1, in accordancewith a preferred embodiment of the present invention;

FIG. 4A is a schematic block diagram illustrating handover of a mobilestation from a CDMA base station to a GSM base station in the system ofFIG. 1, in accordance with a preferred embodiment of the presentinvention;

FIG. 4B is a schematic block diagram illustrating signal flow associatedwith the handover of FIG. 4A, in accordance with a preferred embodimentof the present invention;

FIGS. 4C and 4D are block diagrams that schematically illustratecommunication frames used by the mobile station in carrying out thehandover of FIG. 4A, in accordance with a preferred embodiment of thepresent invention;

FIGS. 5A and 5B are flow charts that schematically illustrate operationof the mobile station in performing the handover of FIG. 4A, inaccordance with a preferred embodiment of the present invention;

FIGS. 6A and 6B are flow charts that schematically illustrate operationof the CDMA base station in performing the handover of FIG. 4A, inaccordance with a preferred embodiment of the present invention;

FIG. 7 is a schematic block diagram illustrating signal flow associatedwith provision of time of day information in the system of FIG. 1, inaccordance with a preferred embodiment of the present invention;

FIG. 8 is a schematic illustration showing cells in a hybrid GSM/CDMAcellular communications system, useful in understanding a method forhandover of a mobile station from a GSM base station to a CDMA basestation, in accordance with a preferred embodiment of the presentinvention;

FIG. 9 is a schematic block diagram illustrating signal flow associatedwith a handover of a mobile station from a GSM base station to a CDMAbase station, in accordance with a preferred embodiment of the presentinvention;

FIGS. 10A and 10B are flow charts that schematically illustrateoperation of the mobile station in performing the handover of FIG. 8, inaccordance with a preferred embodiment of the present invention;

FIG. 11 is a flow chart that schematically illustrates operation of theCDMA base station in performing the handover of FIG. 8, in accordancewith a preferred embodiment of the present invention;

FIG. 12 is a schematic block diagram illustrating handover of a mobilestation between CDMA base stations in a hybrid GSM/CDMA cellularcommunications system, in accordance with a preferred embodiment of thepresent invention;

FIG. 13 is a schematic illustration showing signal flow associated withthe handover of FIG. 12, in accordance with a preferred embodiment ofthe present invention; and

FIGS. 14A-14D are schematic block diagrams illustrating CDMA long codesallocated in conjunction with the handover of FIG. 12, in accordancewith a preferred embodiment of the present invention.

FIG. 15 is an illustration of a flowchart showing a process that takesplace when a multicarrier base station wishes to determine whether itmight be beneficial to perform a handover to a GSM base station.

FIG. 16 is an illustration of a flowchart showing a process that takesplace when a multicarrier base station wishes to determine whether itmight be beneficial to perform a handover to a direct spread basestation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Overview of HybridGSM/CDMA System Operation

Reference is now made to FIG. 1, which is a schematic block diagram of ahybrid GSM/CDMA cellular communications system 20, in accordance with apreferred embodiment of the present invention. System 20 is built arounda public land mobile network (PLMN) 22, which is based on the GSMcommunications standard, as described hereinabove. Infrastructure forsuch networks already exists and is in wide use in many countries, andthe present invention has the advantage of enabling gradual introductionof CDMA service in conjunction with such a network without requiringmajor changes to the existing infrastructure. PLMN 22 comprises at leastone mobile-services switching center (MSC) 24, or possibly a number ofsuch centers (although only one MSC is shown here for clarity ofillustration), which controls network operations within a geographicalarea. Among other functions, MSC 24 is responsible for locationregistration of subscriber units and handover of subscriber unitsbetween base stations, as well as linking PLMN 22 to a public switchedtelephone network (PSTN) and/or packet data network (PDN) 48. The PLMNalso comprises a network management center (NMC) 26 and a cell broadcastcenter (CBC) 28. These functions are described further hereinbelow.

System 20 includes a plurality of mobile stations (MS) 40, whichcommunicate with PLMN 22 via a plurality of base station subsystems(BSS) 30 and 32 over a wireless RF link in one or more of the acceptedcellular communications frequencies. MS 40, which is also known as asubscriber unit, is capable of communicating with both GSM BSS 30, usinga substantially standard GSM TDMA signaling protocol, and CDMA BSS 32,using CDMA-based communication methods described hereinbelow.Additionally, although in standard GSM systems, mobile stations cantypically receive broadcasts from CBC 28 only in idle mode, MS 40 iscapable of receiving such broadcasts during a call through BSS 30, aswill be described further hereinbelow. Although for the sake of clarity,only one each of MS 40, GSM BSS 30 and CDMA BSS 32 is shown in FIG. 1,it will be understood that in actuality, system 20 typically comprises aplurality of each of these system elements.

Both GSM BSS 30 and CDMA BSS 32 communicate with and are controlled byMSC 24. Communications between GSM BSS 30 and MSC 24 are substantiallyin accordance with GSM standards. CDMA BSS 32 is modified relative tothe IS95 CDMA standard so as to communicate with PLMN 22 in accordancewith GSM standards, and particularly so as to communicate with MSC 24via the GSM standard A-interface, as further described hereinbelow withreference to FIGS. 3A and 3B. BSS 32 also communicates with CBC 28, soas to receive messages to be broadcast over the air, and comprises aradio operation and maintenance center (OMC-R) 38. The OMC-Rcommunicates with NMC 26 over a GSM-standard Q3 interface, preferablyusing an information model based on the GSM 12.XX series ofspecifications, which are incorporated herein by reference. Optionally,BSS 32 may be linked to a general packet data service (GPRS) 50, such ashas been proposed by the European Telecommunications Standards Institute(ETSI). Alternatively or additionally, BSS 32 may be coupled fortransmission of packet data directly to PSTN/PDN 48 (although such aconnection is, for the sake of simplicity, not shown in FIG. 1),preferably with a link to the Internet therethrough.

Communications between CDMA BSS 32 and MS 40 are built on a CDMA “airinterface,” which is preferably generally in accordance with the IS95standard for CDMA communications. BSS 32 is built around a base stationcontroller (BSC) 34, which controls and communicates with a number ofbase station transceivers (BTS) 36. Each BTS transmits RF signals to andreceives RF signals from MS 40 when the MS is within a geographicalarea, or cell, served by the particular BTS. When during a telephonecall, the MS moves from the cell of one CDMA BTS 36 to another, a “softhandover” (or handoff) between the BTSs takes place, as is known in theCDMA art.

There may also be regions of service of system 20, however, which do nothave CDMA coverage (i.e., there is no CDMA BTS 36 in such a region), orin which coverage is weak or congested. If MS 40 moves into such aregion during a telephone call, the MS is handed over from the CDMA BTSto a BTS associated with GSM BSS 30 without interrupting the call.Similarly, if MS 40 moves from a region served only by GSM BSS 30 intothe cell of CDMA BTS 36 during a call, the MS is preferably handed overfrom the GSM to the CDMA BSS. Methods for performing such handoversbetween CDMA and GSM/TDMA service and vice versa, as well as between oneCMDA BSS 32 and another, are described further hereinbelow. By virtue ofsuch methods and of the architecture of system 20, as shown in FIG. 1,MS 40 receives the benefits of CDMA service in those regions served bysystem 20 in which the service has been implemented, without losingservice in TDMA regions. Transitions between CDMA and TDMA regions aresubstantially transparent to users of MS 40, because higher-level GSMnetwork protocols are observed throughout the system, and only thelower-level RF air interface is changed during the transition.

FIG. 2A is a block diagram that schematically illustrates communicationsprotocol stacks between MS 40 and BSSs 30 and 32, in accordance with apreferred embodiment of the present invention. MS 40 communicates withGSM BSS 30 over a GSM Um interface, which is based on a standard TDMAair interface, so that substantially no modification is required to BSS30 or to GSM Layer 1 and Layer 2 standard interface protocols in orderto accommodate MS 40. MS 40 communicates with CDMA BSS 32 over a CDMA Uminterface, based on a CDMA IS-95 air interface with certainmodifications. Subscriber units known in the art are capable ofoperating over either a GSM Um or a CDMA Um interface, but not both.

In order to sustain both of these interfaces, MS 40 comprises mobileequipment (ME) 42 (FIG. 1), which must include either two radiotransceivers, one configured for TDMA operation and one for CDMA, or asingle transceiver which can dynamically switch between TDMA and CDMA.The ME includes mobile termination (MT), which supports terminalequipment (TE) 46 for voice and/or data input and output. In addition,MS 40 comprises a subscriber identity module (SIM) 44, in accordancewith GSM standards.

FIG. 2B is a schematic block diagram illustrating MS 40 comprising asingle radio transceiver in ME 42, in accordance with a preferredembodiment of the present invention. MS 40 is built around a modem unit59, including a DSP core 60 capable of generating and processing bothTDMA and CDMA signals. Preferably, core 60 comprises an ASIC device,including stand-alone CDMA transmission/reception processing, which issupported by GSM timing logic 64 and a GSM hardware accelerator (or DSP)62, as well as having a port for SIM 44. Core 60 receives input anddelivers output to TE 46. In this case, TE 46 is represented as an audiomicrophone and speaker, and core 60 performs D/A and A/D conversion, aswell as vocoding functions on the audio signals, as are known in theart. Either GSM or CDMA vocoding is applied, depending on whether MS 40is in contact with GSM BSS 30 or CDMA BSS 32. Core 60 may, additionallyor alternatively, be configured to work with TE 46 providing digitaldata input/output, such as a fax device.

Core 60 outputs digital data, which may be in either TDMA or CDMAformat, to a mixed-signal output device 66. Device 66 processes andconverts the data to analog baseband form, for input to RF transmitter68. A duplexer 70 conveys the resultant RF signals via antenna to theGSM or CDMA base station, as appropriate. Signals received from the basestation are passed by duplexer 70 through an RF receiver 72 and amixed-signal input device 74, which performs baseband conversion and AGCfunctions, to core 60. Preferably, transmitter 68, receiver 72 andmixed-signal devices 66 and 74 are controlled by core 60.

RF transmission and reception by MS 40 are preferably at frequencies inthe GSM 900 or 1800 MHz band, for compatibility with existing GSMequipment, particularly BSS 30. Assuming that MS 40 includes only thesingle transceiver shown in FIG. 2B, operating in the GSM band, CDMAequipment in system 20 must be appropriately configured to operate inthis frequency range, as well.

Returning to FIG. 2A, whether MS 40 physically includes one transceiveror two, it must support dual air interface Layers 1 and 2 in itsprotocol stack, for operation vis-a-vis GSM BSS 30 and CDMA BSS 32,respectively. The CDMA air interface between MS 40 and CDMA BSS 32comprises CDMA Layer 1, which operates on a standard IS-95 protocol, andGSM-CDMA Layer 2, in which IS-95 operation is modified to accommodatethe needs of GSM network services. GSM-CDMA Layer 2 includesfunctionality, such as message ordering, priority and fragmentation, andsuspension and resumption of communications, which is normally supportedby the standard GSM Layer 2, but not by CDMA IS-95. Vis-a-vis GSM BSS30, air interface Layers 1 and 2 are in accordance with GSM standards,substantially without modification.

Standard GSM protocols include a third Radio Interface Layer (RIL3),including three sub-layers, above GSM Layer 1 and Layer 2. The lowest ofthese three RIL3 sub-layers is a Radio Resource (RR) management layer,which supports Mobile Management (MM) and Connection Management (CM)sub-layers above it. The RIL3 sub-layers in GSM BSS 30 are substantiallyunchanged with respect to the GSM standard, and the GSM MM and CMsub-layers are likewise maintained substantially without change in MS40. The CM sub-layer supports signaling for call processing, as well asGSM supplementary services and short message service (SMS). The MMsub-layer supports signaling required for locating MS 40, authenticationand encryption key management.

In order to support the MM and CM sub-layers, a GSM-CDMA RR sub-layer isintroduced in the MS 40 and BSS 32 protocol stacks. The GSM-CDMA RRsub-layer, which manages radio resources and maintains radio linksbetween MS 40 and BSSs 30 and 32, is “aware” of the existence of thedual GSM and CDMA lower layers (Layers 1 and 2) in the MS 40 protocolstack. It invokes the appropriate lower layers in the MS stack tocommunicate with either the standard RIL3-RR sub-layer of BSS 30 overthe GSM Um interface or the GSM-CDMA RR sub-layer of BSS 32 over theCDMA Um interface, depending on instructions it receives from the BSSwith which it is in communication. The MM and CM sub-layers are notprocessed by BSS 32, but are rather relayed through between MS 40 andMSC 24 for processing in a manner substantially transparent to the CDMAair interface layers below. The RR sub-layer in the MS stack alsocontrols the handover between the corresponding air interfaces definedin Layers 1 and 2 and assists in cell selection for the handover, underinstructions from MSC 24 and the BSSs.

Regardless of which of the air interfaces is in use, the GSM-CDMA RRsub-layer supports the standard GSM RIL3-MM and CM sub-layers above it.The RR sub-layer preferably offers complete radio resource managementfunctionality as defined by GSM specifications 04.07 and 04.08, whichare incorporated herein by reference. Although a “RR” layer per se isnot defined by the CDMA IS-95 standard, the GSM-CDMA RR sub-layerdescribed herein maintains full IS-95 radio resource functionality, aswell.

In accordance with GSM standards, the functionality of the RR sub-layerincludes both idle mode operation and dedicated mode services (i.e.,services performed during a telephone conversation). The idle modeoperation of the RR sub-layer includes automatic cell selection and idlehandover between GSM and CDMA cells, as well as between pairs of CDMAcells and pairs of GSM cells, with cell change indication as specifiedby the GSM standard. The RR sub-layer in idle mode also performsbroadcast channel processing, as specified by GSM and CDMA standards,and establishment of RR connections.

In dedicated mode, the RR sub-layer performs the following services:

-   -   Routing services, service request, transfer of messages, and        substantially all other functions specified by GSM standards.    -   Change of dedicated channels (handover), including hard        handovers as described hereinbelow and CDMA-to-CDMA soft and        “softer” handovers.    -   Mode settings for the RR channel, including transmission mode,        type of channel and coding/decoding/transcoding mode.    -   MS parameters management based on IS-95 specifications.    -   MS classmark management based on GSM specifications.

It will be understood by those skilled in the art that the abovefeatures of the RR sub-layer are listed only by way of a summary, andthat additional details and features may be added based on published GSMand CDMA specifications.

FIG. 3A is a block diagram that schematically illustrates protocolstacks used in signaling interfaces between MS 40, CDMA BSS 32 and GSMMSC 24, in accordance with a preferred embodiment of the presentinvention. These interfaces enable MS 40 to communicate with GSM MSC 24over a CDMA air interface. Operation of these interfaces, andparticularly message flow through these interfaces, is described ingreater detail in the above-mentioned PCT patent applicationPCT/US96/20764 and incorporated herein by reference. When MS 40 is incommunication with MSC 24 via GSM BSS 30, the protocol stacks are inaccordance with GSM standards, substantially without modification.

As noted hereinabove, MS 40 exchanges signals with CDMA BSS 32 over theCDMA Um interface, wherein the MS and BSS protocol stacks are modifiedto include the GSM-CDMA RR sub-layer and Layer 2. In FIG. 3A, a relaylayer is shown explicitly in the BSS 32 protocol stack, for conveyingRIL3-CM and MM signaling between MS 40 and MSC 24, largely withoutprocessing by BSS 32. Other layers involved in the Um interface weredescribed hereinabove with reference to FIG. 2A.

CDMA BSS 32 communicates with GSM MSC 24 over a standard, substantiallyunmodified GSM A-interface. This interface is based on the GSM SS7 andBSS Application Part (BSSAP) protocols, as are known in the art,preferably in accordance with the GSM 08.08 standard. BSSAP supportsprocedures between MSC 24 and BSS 32 that require interpretation andprocessing of information related to single calls and resourcemanagement, as well as transfer of call control and mobility managementmessages between MSC 24 and MS 40. BSS 32 translates CDMA Layer 1 andGSM-CDMA Layer 2 and RR protocols exchanged between the BSS and MS 40into appropriate SS7 and BSSAP protocols for transmission to MSC 24, andvice versa.

Because CDMA BSC 34 communicates with GSM MSC 24 using the standardA-interface, substantially no modifications are required in the core GSMMSC in order to enable the addition of CDMA BSS 32 to GSM system 20.Furthermore, MSC 24 need not be aware that there is any difference inidentity between GSM/TDMA BSS 30 and CDMA BSS 32, since both communicatewith the MSC in a substantially identical manner over the A-interface.Preferably, cells associated with BTSs 36 of BSS 32 are mapped by MSC 24in substantially the same manner as GSM/TDMA cells, and are thusassigned GSM absolute radio frequency channel number (ARFCN) and basestation identity code (BSIC) values, in accordance with the GSMstandard. From the point of view of MSC 24, a handover between GSM BSS30 and CDMA BSS 32, or even between two different CDMA BSSs, is nodifferent from a handover between two GSM BSSs in a conventionalGSM/TDMA-based system. The BSIC of the CDMA cells is assigned so as tobe distinguishable within system 20 from conventional GSM cells.

FIG. 3B is a block diagram that schematically illustrates protocolstacks involved in conveying voice data between MS 40 and MSC 24 viaCDMA BSS 32, in accordance with a preferred embodiment of the presentinvention. Voice data between MS 40 and BSS 32 are coded and decoded bya CDMA vocoder, which may comprise any of the standard IS-95 vocoderprotocols known in the art. BSS 32 translates CDMA Layer 1 into GSM ElTDMA signals, and converts the CDMA vocoded data into PCM A-lawcompanded voice data, in accordance with the requirements of theA-interface standard. MSC 24 thus transmits and receives voice data toand from MS 40 via BSS 32 substantially without regard to the fact thatthe data between the BSS and the MS are CDMA-encoded, as though MS 40were operating in GSM/TDMA mode.

CDMA to TDMA Base Station Handover

FIG. 4A is a schematic block diagram showing details of system 20,useful in understanding a method for mobile-assisted handover of MS 40from CDMA BSS 32 to GSM BSS 30, in accordance with a preferredembodiment of the present invention. Unlike FIG. 1, BSS 30 is shown herein detail to include a BSC 77 and a plurality of BTSs 78 and 80. FIG. 4Aillustrates the handover of MS 40 from one of the BTSs associated withBSS 32, labeled here BTS 76, to BTS 78 of BSS 30. BSS 32 also includesGSM-CDMA BSC 34 and BTSs 36, as described with reference to FIG. 1.

The handover from CDMA BTS 76 to TDMA BTS 78 is preferably initiated byBSS 32 when it is determined that MS 40 is in a location in which such ahandover might be desirable. This situation may arise when the signalreceived from BTS 76 is weak, or when MS 40 is known to be reaching theedge of a CDMA coverage area, or when traffic on CDMA channels is heavy.Alternatively, BSS 32 may instruct MS 40 to seek a signal from BTS 78(or other GSM BTSs) from time to time independently of any specificpressure to do so.

FIG. 4B is a schematic signal flow diagram, illustrating signalsconveyed between MS 40, BSSs 30 and 32 and MSC 24 in the handoverprocess of FIG. 4A, in accordance with a preferred embodiment of thepresent invention. BSC 34 instructs MS 40 to begin a gated search forneighboring GSM BTSs, wherein for brief periods, MS 40 interrupts itscommunications with BTS 76 to search for and receive TDMA signals.Preferably, MS 40 is operating on the IS95 standard, which enables CDMAtransmission to be idle for the duration of a 20 ms frame, during whichthe GSM TDMA neighbor scan can take place without substantiallyinterrupting CDMA voice communications. Most preferably, transmission byMS 40 during the 20 ms frame is suspended using anactivation/deactivation mechanism as defined by the IS-95B standard,section 6.6.6.2.8. Alternatively, such an idle period may also beintroduced under other CDMA standards, as well. Further alternatively,as noted hereinabove, MS 40 may comprise separate TDMA and CDMAtransceivers that can be used simultaneously for this purpose.

Preferably, BSC 34 provides MS 40 with a list of the frequencies ofneighboring GSM TDMA cells, such as those associated with BTSs 78 and80. Such a list is useful in reducing the time needed to search for andfind BTS 78, since MS 40 will search only at the frequencies of thecells on the list. The list is updated as MS 40 moves from one cell toanother and is maintained during handovers between TDMA and CDMA basestations.

When MS 40 receives a signal at the frequency of BTS 78, it attempts todecode the GSM frequency correction (FCCH) and synchronization (SCH)channels in the signal. This decoding may take several of the gated CDMAidle periods to complete. Once decoding is successfully accomplished, MS40 determines the power level of the TDMA signal and reports it to BSS32 together with the GSM cell identity. To determine the power level, MS40 preferably averages the signal power over a period, so as to reducethe influence of MS movement and channel fading. The determination andreporting of the TDMA power level is preferably repeated continuallyafter MS 40 has received the command to do so.

In accordance with GSM standards, the power level for each cellmonitored by MS 40 should be determined at least once every 5 sec, andthe corresponding SCH should be decoded at least once every 30 sec. Thepower levels should be determined for all of the cells on the list ofneighboring cells provided by BSS 32. Preferably, the MS decodes the SCHand reports the power level only of the cell from which the best signalwas received. Most preferably, the MS reports to BSS 32 only when therehas been a change in the determined power level since the last report orsome other change of significance in the signals received by the MS fromthe monitored cells.

Based on this information, the BSS determines whether and when ahandover is to take place. At an appropriate time, BSS 32 initiates ahandover request to MSC 24. The MSC conveys the handover request to GSMBSS 30, which acknowledges the request. GSM BSS 30 then conveys a RRhandover command via MSC 24 and CDMA BSS 32 to MS 40, and a new trafficchannel (TCH) is opened between BSS 30 and the MS. At this point thehandover is complete, and MS 40 switches over to BTS 78. A successfulhandover is reported to MSC 24, substantially in accordance with GSMmessaging standards, following which the MSC issues a suitable “clear”command to CDMA BSS 32, which responds with a “clear complete” message.

Preferably, the new traffic channel is opened in a non-synchronizedhandover mode, in accordance with accepted GSM handover methods, and GSMBSS 30 is configured to accept such a handover. MS 40 preferablyresponds to the RR handover command with a handover access burst on themain dedicated control channel (DCCH) of GSM BSS 30, as indicated by thehandover command. The MS then waits to receive an appropriate physicalinformation message from BSS 30 on the TCH, as defined in GSM standard04.08, in order to complete the handover. If the physical information isnot received within a predetermined time period, preferably within 320ms, in accordance with the T3124 timer of the IS-95 standard, the MSattempts to resume its connection to CDMA BSS 32.

The decision to initiate the handover may take place whenever the signalfrom GSM BTS 78 becomes stronger than that of CDMA BTS 76, butpreferably other criteria are applied. For example, since CDMA channelstypically offer better transmission quality than GSM channels, thehandover is preferably initiated only when the GSM signal is strongerthan the CDMA signal by some predetermined weighting factor. The factormay be preprogrammed in system 20, or it may be set by a user of MS 40.It may also be adjusted dynamically in response to such parameters asthe geographical location of the MS and the relative amounts of trafficon the CDMA and TDMA channels in the system.

FIGS. 4C and 4D are block diagrams that schematically illustrate thestructure of IS-95B frames 81 and 87, used respectively by MS 40 todecode and monitor the power of TDMA cells, in accordance with apreferred embodiment of the present invention. Monitoring frames 81 and87 are interspersed with normal CDMA communication frames 82, at arepetition rate of no more than one monitoring frame in 480 ms. IS-95Bstandards allow the monitoring frames to have a duration of either 20 msor 40 ms. Longer monitoring periods may be used if desired. The choiceof shorter (20 ms) frames decreases possible data loss in a CDMA callbeing conducted simultaneously between MS 40 and BSS 32, although itincreases the length of time needed to complete a cycle of decoding andmonitoring.

FIG. 4C illustrates monitoring frame 81, which is used to acquire theFCCH and SCH of a particular TDMA cell of interest. In an initialinterval 83, MS 40 adjusts its receiver frequency, typically byadjusting an appropriate phase-locked loop (PLL) to the frequency of theTDMA cell. In a subsequent interval 84, the MS adjusts its receiver gainfor the signal being received from the TDMA cell, typically usingautomatic gain control (AGC). Suitable methods of PLL and AGC adjustmentare well known in the art. Intervals 83 and 84 are preferably about 1 mseach in duration. Subsequently, for about 15 or 35 ms, depending onwhether the total duration of frame 81 is 20 or 40 ms, the FCCH and SCHof the acquired TDMA cell are decoded, as described hereinabove. Then,in preparation for the next CDMA frame 82, MS 40 readjusts its frequencyto its previous (CDMA) setting and then resynchronizes to CDMA BTS 76 ina final interval 86.

FIG. 4D illustrates monitoring frame 87, which is used to measure powerlevels of TDMA cells of interest. For each such cell, the frequency ofMS 40 is adjusted in an initial interval 83, as described above. Thecell power level is then determined during a corresponding energymeasurement interval 88, preferably having a duration of about 1.4 ms.In the example shown in FIG. 4D, the duration of frame 87 is taken to be20 ms, enabling power levels to be determined for seven different cellsduring the frame. Alternatively, if a 40 ms frame is used, the powerlevels of up to 15 different cells may be determined during the frame.

In an alternative embodiment not shown in the figures, one monitoringframe may be divided into two or more parts, one for acquiring the FCCHand SCH, and the other for energy measurements. Further alternativeembodiments may be based on the IS-95C or IS-95Q CDMA standard.

FIGS. 5A, 5B, 6A and 6B are flow charts that schematically illustrate,in the form of state machines, operations involved in performing thehandover illustrated in FIGS. 4A and 4B, in accordance with a preferredembodiment of the present invention. FIGS. 5A and 5B illustrate statesof MS 40, and FIGS. 6A and 6B illustrates states of GSM-CDMA BSS 32.Solid lines in these figures represent processes carried out using IS-95gating, as described hereinabove, so that the MS switches between CDMAand TDMA reception. Dashed lines indicate alternative state transitionsthat are possible when the MS is capable of simultaneous CDMA/TDMAoperation, typically requiring that the MS have dual radio transceivers(unlike the single-transceiver MS shown in FIG. 2B). The states ofGSM-TDMA BSS 30 are not shown, since they are substantially inaccordance with GSM standards, which are known in the art.

Certain messages conveyed between MS 40 and BSS 30 and BSS 32 in thecourse of the handover processes are indicated along the linesconnecting relevant states of BSS 32 and MS 40 in the figures. Thesemessages preferably have the general form of standard IS-95 or GSMmessages, as appropriate, which are modified and/or supplemented so asto carry additional information that needs to be conveyed in hybridGSM-CDMA system 20. Although certain exemplary messages and messageformats are described herein, substantially any suitable assignment ofthe message fields may be used, within the constraints of the relevantIS-95 and GSM standards, as will be clear to those skilled in the art.

At the start of the handover process, MS 40 is in communication with BSS32 over a CDMA traffic channel (TCH) in a state 100 of the MS and astate 130 of the BSS. The BSS issues a search gating command, includinggating parameters, and then waits for gating completion in a state 134.MS 40 checks the parameters in a state 102. If the MS is not configuredto support the parameters, it issues a gating reject message. If theparameters are supported, the MS issues a gating complete message andenters an IS-95 gating state 104. If a stop gating command is received,MS 40 returns to state 100.

Upon receiving the gating complete message, BSS 32 enters an IS-95gating state 136 and commands MS 40 to start monitoring neighboringcells. (As noted above, gating states 104 and 136 are not needed if theMS is capable of simultaneous CDMA/TDMA operation, in which case the MSenters state 106 directly from state 100.) The BSS then enters a state132 in which it waits for the monitoring to be completed. The MS checksthe monitoring command parameters in a state 106. Having verified thatit does support the monitoring command parameters, MS 40 enters a GSMmonitoring state 108, in which it periodically decodes and determinesthe signal strength of the neighboring cells, as described hereinabove.Likewise, upon receiving confirmation from the MS that it has begunmonitoring neighboring cells, BSS 32 enters a respective GSM monitoringstate 138.

MS 40 continues monitoring the neighboring cells and reporting theresults to BSS 32 in the form of a pilot strength measurement message(PSMM). When a handover trigger condition is established, i.e., when thesignal received by MS 40 from BSS 32 is sufficiently weaker than one ofthe neighboring cells, the BSS indicates to MSC 24 that a handover isrequired and enters a waiting state 140. If no handover command isreceived within a predefined period, preferably determined by GSM timerT7, in accordance with the GSM standard, the BSS returns to state 138.When the handover command is received from the MSC, BSS 32 passes theRIL3-RR handover command on to MS 40, and then enters another waitingstate 142, where it awaits a Layer 2 (L2) acknowledgment of the commandfrom the MS. It is noted that BSS 32 may also receive a handover commandwhile in state 138, in which case it similarly issues the RIL3-RRhandover command to MS 40 and enters state 142.

When MS 40 receives the RIL3-RR handover command, it checks the handovercommand parameters in a state 110. If MS 40 supports the handovercommand parameters, it sends the L2 acknowledgment to BSS 32 and entersa CDMA suspension state 112. If the parameters are not supported, MS 40issues a handover failure message and returns to state 108. In thiscase, or if no acknowledgment is received within a predefined period,preferably determined by GSM timer T8, BSS 32 sends a handover failuremessage to MSC 24 and returns to state 138.

Assuming that the parameters are supported, and that the handovercommand indicates that the MS is to be handed over to GSM-TDMA BSS 30,the MS sends the handover access message and then waits in a state 120for the physical information from BSS 30. (If the handover commandspecifies that the MS is to be handed over to another CDMA BSS, the MSenters a state 114, as described further hereinbelow with reference toFIGS. 12 and 13.) Meanwhile, BSS 32 awaits a “clear” command in a state144, while periodically sending “clear request” messages to MSC 24.

Once the physical information has been received, the handover issuccessfully completed, and MS 40 enters a GSM traffic channelcommunications state 124. BSS 32 receives the clear command, whereuponit enters a state 148, in which it releases air resources allocated tothe communication channel with MS 40 and sends a “clear complete”message. The BSS enters a SCCP release state 150, in which it releasescall resources used in communicating with MSC 24, and then ends itsconnection with MS 40 in an end state 152.

If MS 40 does not receive the physical information within a specifiedperiod, however, given by expiration of the GSM T3124 timer, the MSenters a state 122 in which it attempts to reacquire CDMA BSS 32 andreturn to state 100. A handover failure message is issued to BSS 32,which then enters a corresponding CDMA reacquisition state 146. Ifreacquisition is unsuccessful, BSS 32 issues a clear request and returnsto state 144, from which it may ultimately exit to state 152, asdescribed above. The MS transfers to an idle state 126.

TDMA to CDMA Base Station Handover

FIG. 7 is a schematic block diagram showing signal flow in system 20(FIG. 1) associated with providing the time of day to relevant GSM BSCsand BTSs in the system, in accordance with a preferred embodiment of thepresent invention. Ordinarily, GSM BSSs in system 20 would not beinformed of the time of day, since this information is not required bythe GSM standard. On the other hand, the IS-95 standard requires thatCDMA base stations be synchronized, since such synchronization isnecessary for identification and decoding of the signals and for softhandover between cells. Therefore, for mobile-assisted handover of MS 40from TDMA BTS 78 to CDMA 76 (as shown in FIG. 4A, but with the directionof the handover arrow reversed), it is necessary that the time of day beprovided by system 20.

The method of FIG. 7 allows the time of day to be provided in system 20without the necessity of hardware or software changes in MSC 24 or inGSM BSS 30 or BTSs 78 and 80, by using CBC 28, which is a standard partof PLMN 22, to broadcast the time of day over the system. Ordinary, CBC28 provides a cell broadcast service (CBS) in accordance with GSMinterface standards 03.41 and 03.49, enabling general short messages tobe broadcast unacknowledged to defined geographical areas within system20. The messages are received by MS 40 while it is in standby, or idle,mode, i.e., when the MS is not involved in a telephone call. For thepurpose of providing time of day information, however, MS 40 ispreferably capable of receiving CBS messages not only when it is in anidle mode, as prescribed by GSM standards, but also when the MS is in adedicated mode, i.e., during a telephone call (although at the possibleexpense of losing data from the call itself). The use of the CBS toprovide time-of-day information to MS 40 is desirable particularly whenthe MS includes only a single radio transmitter and receiver, as shownin FIG. 2B; when dual radios are used, one for CDMA and the other forTDMA, the CDMA radio can receive the time of day while the TDMA radio isin use in a telephone call.

In a preferred embodiment of the present invention, CBS messages arealso used to initiate a search by MS 40 for neighboring cells, asdescribed above with reference to FIG. 4B.

A special MS 160, which is equipped with a GPS (global positioningsystem) receiver 161, is located in one or more of the GSM/TDMA cells ofsystem 20 in which the time of day is needed. In FIG. 7, MS 160 receivesthe time of day from receiver 161 and associates the time with anidentification of the concurrent TDMA frame number, based onsynchronization signals transmitted by BTS 78, in accordance with theGSM standard. Alternatively, MS 160 may be configured to receive thetime of day from a CDMA BSS, in which case GPS receiver 161 is notrequired. MS 160 opens a data call via BTS 78, BSC 77, MSC 24 andPSTN/PDN 48 to CBC 28, and sends to the CBC the cell identification andcorrespondence of the current time of day and frame number.Alternatively, MS 160 may convey the information by any other suitablemethod, such as using the GSM SMS. CBC 28 then transmits thisinformation over the CBS to the cell, so that MS 40 receives the time ofday even when it is operating in GSM/TDMA mode. Therefore, when MS 40 isto be handed over to CDMA BTS 76, there is no need to acquiresynchronization/time of day information from the CDMA BTS, and thehandover can proceed more rapidly and smoothly.

Introducing the time of day into system 20 also has benefits for the GSMportion of the system in itself, without connection to CDMA handover.For example, MS 40 can transmit its time of day to different GSM BTSs 78and 80, and the timing delay from the MS to each of the BTSs can bemeasured and used to determine the location of the MS.

FIG. 8 is a schematic map of overlapping GSM/TDMA cells 162 and CDMAcells 164 in network 20, illustrating aspects of mobile-assistedhandover from GSM BTS 78 to CDMA BTS 76, in accordance with a preferredembodiment of the present invention. An operator of system 20 willrecognize that when MS 40 is located in any of cells 1-5 shown in FIG.8, a TDMA/CDMA handover may take place. Therefore, CBC 28 will broadcasta CBS message to all dual-mode (GSM/CDMA) MSs in these cells, includingthe following information and instructions:

-   -   MS to begin search for CDMA signals (search trigger).    -   Frequencies of CDMA BTSs in overlapping and neighboring cells.    -   GSM mapping of CDMA cells 94, according to GSM MSC 24.    -   Identification of the time of day with the current TDMA frame        number, preferably as derived from MS 90, although other methods        may also be used to supply the time of day.    -   Optionally, the factor by which the CDMA signal strength is to        be multiplied for comparison with the TDMA signal, as described        hereinabove.

There is no need for such a message to be broadcast in cells 6-10.Furthermore, it will be understood that only the dual-mode MSs areprogrammed to receive and interpret this message, while ordinaryGSM/TDMA MSs will ignore it. The CBS message triggers and enables thedual-mode MSs to gather and provide information to GSM BSS 30 and MSC 24for assistance in making the handover to one of the CDMA BSSs, unlikehybrid GSM/CDMA systems that have been suggested in the prior art.

FIG. 9 is a block diagram illustrating signal flow in system 20associated with a mobile-assisted handover from BTS 78 to BTS 76, inaccordance with a preferred embodiment of the present invention. Asnoted above with reference to FIG. 7, the handover begins with thetransmission of the search trigger and other information. The searchtrigger is transmitted periodically by BTS 78 whenever MS 40 is in oneof GSM cells 1-5 (FIG. 8), or in response to some other preprogrammedcondition.

Upon receiving the trigger, MS 40 switches off its TDMA traffic with BTS78 and tunes its receiver to an appropriate CDMA frequency for a shortperiod, preferably for about 5 ms. Then, after the MS has resumedcommunicating with BTS 78, it attempts to decode any CDMA signal itreceived in order to identify a pilot beam of the BTS whose transmissionit has received, say from BTS 76. As noted above, CDMA BTS 76 is mappedin system 20 as though it were a GSM-TDMA BTS. MS 40 therefore transmitsa report message back to GSM BTS 78 indicating the power of the signalit received from BTS 76 (optionally multiplied by the relative CDMA/TDMAweighting factor mentioned above), together with the GSM system mapidentification of BTS 76. From the point of view of GSM BSS 30 and MSC24, there is no substantial difference between the message transmittedby MS 40 in this case and the message that would be transmitted as theresult of an ordinary GSM neighbor scan.

This process of measurement and reporting goes on until BSS 30 adetermines that MS 40 should be handed over to BTS 76. At this point,BSS 30 conveys a message to MSC 24 indicating that the handover isrequired. MSC 24 passes a handover request on to BSS 32, which sends anacknowledgment back via MSC 24 to BSS 30. BSS 32 allocates hardware andsoftware resources to the communications traffic channel to be openedwith MS 40 and begins sending null data to the MS in order to open thechannel. GSM BSS 30 then gives a handover command to MS 40, preferably aRIL3-RR command that encapsulates IS-95 parameters required for openinga CDMA traffic channel with CDMA BTS 76. The parameters contained insuch a message are described further hereinbelow with reference to FIGS.13 and 14A-D. The new traffic channel is then opened, completing thehandover, and BSS 30 releases the old TDMA traffic channel.

The process described above thus allows mobile-assisted handover fromGSM/TDMA BSS 30 to CDMA BSS 32 with high speed and reliability, and withminimal interruption to service in the middle of a call during which thehandover takes place. For the purposes of this handover, GSM cells insystem 20 receive time of day information, and the CDMA cells are mappedinto the GSM system, at minimal hardware expense and substantiallywithout the necessity of reprogramming existing GSM system elements.

A similar TDMA-CDMA handover process may be carried out even in theabsence of time-of-day information at GSM BSS 30. In this case, after MS40 has acquired a pilot channel signal associated with BTS 76, it musttune in to and decode the CDMA sync channel of the BTS in order toderive the time of day. This operation takes about 480 ms, creating anoticeable but still tolerable interruption in voice service during acall. Further alternatively, a similar handover process can be performedusing a MS having two transceivers, one for TDMA and the other for CDMA,as described hereinabove.

FIGS. 10A, 10B and 11 are flow charts that schematically illustrate, inthe form of state machines, the operation of MS 40 and BSS 32 inperforming the handover illustrated in FIG. 9, in accordance with apreferred embodiment of the present invention. FIGS. 10A and 10B referto MS 40, while FIG. 11 refers to BSS 32. BSS 30 operates substantiallyin accordance with GSM standards, as are known in the art.

MS 40 begins in an initial state 170, in which the MS is incommunication with BSS 30, in a certain cell associated with the BSS,over a GSM traffic channel (TCH). When the MS moves into a new cell, itenters a state 172 in which it receives and reads messages from CBC 28.If there is no CBC message preparing MS 40 for possible handover to aCDMA BSS (because there is no CDMA BSS in the area, for example), the MSreturns to a GSM TCH state 174, from which it may be handed over toanother GSM-TDMA BSS.

When prompted by an appropriate CBC message, MS 40 enters an overlaystate 176, in which it acquires the time of day, as described above, andsends pilot strength measurement messages (PSMM) to BSS 30. In standardGSM-TDMA operation, there is generally a free time slot of 6 msavailable once every 120 ms. During these free time slots, MS 40interrupts TDMA transmission to search for pilot beams of neighboringGSM-CDMA cells, such as those associated with BSS 32. If no pilot isfound, the MS goes to a state 180, in which it adjusts its frequency andattempts to find an appropriate GSM frequency correction channel (FCCH).Alternatively, when a pilot is found, the MS enters a state 182, inwhich it adjusts its frequency as required and measures the CDMA signalstrength. During subsequent slots, while MS 40 is communicating over itscurrent GSM-TDMA traffic channel, it attempts to decode the CDMA pilotso as to identify the cell with which the pilot is associated. Theresults are reported to BSS 30.

At an appropriate time, based on the results reported by MS 40, asdescribed above, MSC 24 conveys the handover request to BSS 32. The BSSenters a preparatory state 190, in which it allocates resources, assignsa long code and makes an SCCP connection with the MSC in preparation forhandover. After sending the appropriate acknowledgment message to theMSC, BSS 32 enters a state 191 in which it sends null forward trafficframes to MS 40 and waits to receive reverse traffic from the MS. If theBSS fails to allocate the resources, however, it reports a handoverfailure and exits to an end state 197.

Based on parameters encapsulated in the acknowledgment message from BSS32, the RIL3-RR handover command message is sent from GSM-TDMA BSS 30 toMS 40, identifying the GSM-CDMA destination cell associated with BSS 32and conveying the necessary handover parameters. MS 40 enters a state183 in which it verifies that the handover parameters are supported and,if the verification succeeds, suspends its GSM-TDMA operation in a state184. (If the verification fails, the MS reports the failure and returnsto state 176.) The MS then enters a state 185 in which it waits toreceive a predetermined number of “good” frames, preferably the numberdetermined by the IS-95 counter N11m, from BSS 32. When the good frameshave been received, the MS sends back to the BSS a number of preambleframes (short, dummy frames used in establishing the traffic channel),as specified by the NUM_PREAMBLE parameter in the handover commandmessage, and enters a service option adjustment state 186. BSS 32detects the preamble frames and reports to the MSC that the CDMA trafficchannel has been established, after which the BSS enters a state 192 inwhich it waits for completion of the handover.

If MS 40 and BSS 32 are unable to establish communications, the handoverto BSS 32 is aborted, and MS 40 and BSS 32 return to their earlierstates. MS 40 attempts to reacquire GSM BSS 30 in a state 188 and, ifsuccessful, returns to GSM TCH state 170. If the reacquisition fails,the MS exits to an idle mode 189. In either case, BSS 32 receives aclear command, and releases all resources it had allocated to MS 40 in astate 193, following which BSS 32 exits to end state 197.

Assuming the handover is successfully completed, however, BSS 32 entersa service option adjustment state 194, corresponding to state 186 of MS40. A service request is issued by BSS 32, and the BSS awaits a serviceresponse from MS 40 in a waiting state 195. When the service response isreceived, MS 40 and BSS 32 enter respective CDMA traffic channel (TCH)states 187 and 196, and the call continues normally over the CDMAchannel.

CDMA to CDMA Base Station Handover

FIG. 12 is a schematic block diagram illustrating handover between twodifferent CDMA BSSs 201 and 203 within system 20, in accordance with apreferred embodiment of the present invention. BSS 201 comprises a BSC202 and a plurality of BTSs 206 and 208; and BSS 203 comprises a BSC 204and a plurality of BTSs 210 and 212. BSSs 201 and 203 are substantiallysimilar to and interchangeable with BSS 32, shown in FIG. 1 anddescribed hereinabove, and communicate with GSM MSC 24 via the GSMA-interface. MS 40 is shown in the figure in the midst of a handoverfrom BTS 208 to BTS 210, under the control of MSC 24. Although thehandover takes place between two CDMA BSSs, from the point of view ofthe system, it is a handover between two GSM BSSs, wherein BTSs 208 and210 are respectively mapped by MSC 24 as GSM cells.

FIG. 13 is a schematic diagram illustrating signal flow between theelements of system 20 shown in FIG. 12 in the course of the handover, inaccordance with a preferred embodiment of the present invention. Priorto initiating the handover, BSS 201 issues a search trigger to MS 40,which then searches for CDMA transmission frequencies of neighboringcells, preferably using IS-95 gating, substantially as describedhereinabove. The handover is triggered when MS 40 reports to BSS 201that it is receiving a signal from BTS 210 with a higher power levelthan that of BTS 208.

Upon receiving the report from MS 40, BSS 201 sends a handover-requiredmessage to MSC 24, specifying the GSM cell identity of BTS 210 as thenew cell assignment desired for the handover. The message is generallyin accordance with GSM standards. The CDMA data rate of communicationsbetween the MS and the BSS, which according to IS-95 standards can beeither 8 kbit/sec (rate set 1) or 14.4 kbit/sec (rate set 2), ispreferably conveyed in the message by indicating the IS-95 data ratesrespectively as GSM half-rate and full-rate traffic channels. When theGSM traffic channel rate is conveyed to BSS 203, the BSS interprets therate to select the appropriate IS-95 data rate.

MSC 24 sends a handover request to BSS 203, which responds by sending tothe MSC an acknowledgment that encapsulates a RIL3-RR handover commandmessage, which is passed back to BSS 201. Thus, all of the messages sentbetween BSSs 201 and 203 comply with A-interface requirements, and CDMAparameters associated with IS95 are mapped to corresponding GSMparameters, for example, identification of vocoder type 13K QCELP inCDMA to GSM full rate vocoder. The handover request, acknowledge andcommand are passed on by MSC 24 substantially without change.

After receiving the handover command, old BSS 201 sends the RR handovercommand message to MS 40 so as to effect the handover to new BSS 203.The message to MS 40 encapsulates CDMA parameters required for thehandover, in accordance with IS-95 standards, including but not limitedto the following:

-   -   A new long code mask, preferably allocated by BSS 203 from a        pool of available numbers, in such a manner that mask values        used in a common coverage area are as distant as possible from        one another and that no two MSs in the area have the same mask.        An exemplary scheme of long code mask allocation is described        below with reference to FIGS. 14A-D. Although in standard IS-95        cellular systems, the long code mask of the MS is fixed and is        conveyed to the new BS in the course of the handover, GSM        standards do not provide a message that can be used to convey        the long code mask to new BS 203. It is therefore necessary for        BS 203 to allocate the new long code mask and pass it back to MS        40 via BS 201, preferably in the RR handover command, as        described herein.    -   Nominal power level parameters, preferably NOM_PWR and        NOM_PWR_EXT, as specified by IS-95 standards, providing a        correction factor to be used by MS 40 in open-loop power        estimation, by means of which the MS sets the power level of        signals to be transmitted to BSS 203.    -   Frame offset, a parameter that indicates, preferably in steps of        1.25 ms, a delay of forward and reverse traffic channel frames        sent to and received from MS 40, relative to system timing of        system 20. The frame offset is passed from BSS 201 to BSS 203 in        the handover command message. An optional ACTIVE_TIME parameter        may also be included to indicate the time at which the delay is        to be introduced.    -   Code channel, similarly passed from BSS 201 to BSS 203, to        indicate a Walsh function that is to be used to encode the        forward traffic channel from BSS 203 to MS 40, in accordance        with the IS-95 standard.    -   Layer 2 acknowledgment numbering, which may be used by BSS 203        to reset acknowledgment processing by protocol layer 2 in MS 40,        preferably at a time specified in the handover command message.    -   Forward traffic channel power control parameters, used by BSS        203 to reset the TOT_FRAMES and BAD_FRAMES counts made by MS 40        for the purpose of reporting forward channel error statistics to        the BSS.    -   Number of preamble, indicating the number of preamble frames to        be transmitted by MS 40 to BSS 203 after the MS has received        Nllm good frames from the BSS, as described hereinabove with        reference to FIG. 10B.    -   New band class (frequency range) and frequency (within the        range) of the cell associated with BSS 203 to which MS 40 is now        assigned.

The parameters listed above are not exhaustive and are intended only asa representative sample of the information to be conveyed in thehandover command message. Other IS-95 parameters may similarly beincluded in the message. More generally, those skilled in the art willunderstand how the method exemplified by the handover command describedabove, wherein data associated with one of the air interfaces in system20 (GSM/TDMA or CDMA) are conveyed in messages sent over the other oneof the air interfaces, can be used in a similar manner to conveymessages and data of other types.

After the RR handover command has been sent to MS 40, a new trafficchannel is established between BSS 203 and MS 40. To establish thechannel, BSS 203 sends traffic channel frames to MS 40, which respondswith an appropriate number of preamble frames, as specified by thehandover command message. A successful handover is then reported to MSC24, substantially in accordance with GSM messaging standards, followingwhich the MSC issues a suitable “clear” command to old BSS 201, whichresponds with a “clear complete” message.

FIGS. 14A-D are block diagrams that schematically illustrate 42-bit longcode masks allocated by BSS 203, in conjunction with the handoverillustrated in FIG. 12, in accordance with a preferred embodiment of thepresent invention. FIG. 14A shows a mask 220 for use in an accesschannel; FIG. 14B shows a mask 222 for use in a paging channel; FIG. 14Cshows a mask 224 for use in fundamental (forward and reverse) trafficchannels; and FIG. 14D shows a mask 226 for use in supplementary(forward and reverse) traffic channels. Such supplementary channels areused, for example, in multi-channel medium data rate (MDR)communications, as specified by the IS-95B standard.

Access channel mask 220 preferably comprises an access channel number228, a paging channel number 230, a base station identification number(ID) 232 of BSS 203, and a pilot beam offset 234, all of which areassigned substantially in accordance with IS-95 specifications. Thepaging channel number and pilot beam offset are similarly included inpaging channel mask 222.

Traffic channel masks 224 and 226 represent public long code maskformats. They preferably include base station ID 232 and a unique,16-bit number 236 chosen from a pool assigned to BSS 203. Pool number236 is assigned, as described hereinabove, so that no two MSs can havethe same long code mask. For greater call security, a private long codemask may be used in place of masks 224 and 226. Generation of suchmasks, using a GSM encryption code Kc, is described, for example, in apatent application entitled, “Encryption Support in a Hybrid GSM/CDMANetwork,” filed Oct. 21, 1998, which is assigned to the assignee of thepresent patent application and incorporated herein by reference.

Operation of BSS 201 and BSS 203 in carrying out the handover shown inFIG. 12 can be represented schematically by state machines substantiallysimilar to those illustrated respectively by FIGS. 6A/6B and FIG. 11.Operation of MS 40 in this handover is largely similar to that shown inFIGS. 5A and 5B, up to state 112, at which CDMA communications with BSS201 are suspended. As MS 40 is establishing a new traffic channel withCDMA BSS 203, it passes through states 114, 116 and 118, which areequivalent to states 185, 186 and 187, respectively, as shown in FIG.10B. If MS 40 fails to acquire the new traffic channel while it is instate 114, it passes to state 122, in which it attempts to reacquire oldBSS 201.

The method described hereinabove relates primarily to hard handoversbetween two different BSSs 201 and 203, under the control of MSC 24.System 20 preferably also allows soft handovers of MS 40, in accordancewith IS-95 standards, between BTSs associated with a single BSC, such asBTSs 206 and 208, shown in FIG. 12. Optionally, if BSC 202 is suitablylinked to BSC 204, by a connection generally independent of MSC 24 (notshown in the figures), a soft inter-BSS handover from BTS 208 to BTS 210may also take place. In such cases, BSS 203 informs MSC 24 that thehandover has taken place, so that the new location of MS 40 isappropriately registered.

One of the problems that exists when attempting to measure the amount ofpower that is being transmitted from a GSM system is that the timing ofthe GSM system must be determined. For example, when attempting toperform a handover from a system using a CDMA Multicarrier (MC) airinterface, such as is provided for in third generation CDMA systems,commonly known as “3G” systems, to a GSM system, such as a GSM system,the timing of the GSM system must be determined before powermeasurements can be made and reported. One reason for this is that dueto the frequency reuse schemes used in GSM, it is necessary for the MSmaking measurements to be able to read the Sychronization Channel duringthe time at which a Base Station Identity Code (BSIC) is beingtransmitted. Such BSICs are transmitted roughtly every 10 GSM frames(about every 46 milliseconds). In accordance with GSM industry standardrequirements, the MS must report the BSIC together with the measuredaverage power level (RXLEV) for each GSM signal that is to be measured.One way in which the timing can be determined is by providinginformation to the MS 40 from a MC base station (MC-BS), including theGSM Frame Number, which uniquely identifies the instant in time when theSynchronization Channel is transmitted by a GSM-BSS. It should be notedthat the frame number that is valid at a particular time in one GSM-BSSis not the same as the number that is valid at any other GSM-BSS of thesame system. This is intentionally done in order to allow GSM MSs tomonitor neighboring cells during TDMA idle periods. Therefore, at anyinstant in time the GSM Frame Number is different in each GSM-BSS.

In accordance with one embodiment of the presently disclosed method andapparatus, the information that is provided includes:

(1) CDMA time,

(2) an indication of the number of GSM channels that are to be searched,

(3) a received signal strength threshold, and

(4) information relevant to each of the channels to be searched.

In one embodiment of the disclosed method and apparatus, the informationthat is relevant to each of the channels includes:

(1) the frequency band that includes the channel to be searched,

(2) the frequency of the channel to be searched (such as the “AFRCN”defined in the industry standard related to GSM communication systems),

(3) an identification code associated with the channel (such as the BaseStation Identification Code (BSIC) defined in the industry standardrelated to GSM communication systems),

(4) the frame number (such as the GSM Frame Number defined by theindustry standard related to GSM communication systems) that is beingtransmitted at the identified CDMA time, and

(5) the particular portion of the frame being transmitted at theidentified CDMA time.

In an alternative embodiment of the disclosed method and apparatus, theBSIC is transmitted once for all of the channels to be searched.

The following is a description of how this information is used to reducethe amount of time required to determine whether there is an appropriatecandidate station to which a handover can be made.

FIG. 15 is an illustration of a flowchart showing the process that takesplace when an MC-BS 1501 wishes to determine whether it might bebeneficial to perform a handover. It should be noted that the processshown in FIG. 15 and described below may be performed either in responseto a determination that the signal that is currently supportingcommunications to the MS is too weak, or upon any other triggeringevent.

The process begins with a Candidate Frequency Search Request Message1503 being transmitted from a MC-BS 1501 to a MS 1505. In one embodimentof the disclosed method and apparatus, the Candidate Frequency SearchRequest Message has the following format including the fields shown inTables 1-3:

TABLE 1 Field Length (bits) USE_TIME 1 ACTION_TIME 6 RESERVED_1 4CFSRM_SEQ 2 SEARCH_TYPE 2 SEARCH_PERIOD 4 SEARCH_MODE 4MODE_SPECIFIC_LEN 8 Mode-specific fields 8 × MODE_SPECIFIC_LENALIGN_TIMING 1 SEARCH_OFFSET 0 or 6

In accordance with this embodiment, each of the fields shown is definedby the industry standard for GSM communications systems. However, in oneembodiment of the disclosed method and apparatus, an additional searchmode is defined. This additional search mode requests searches for GSMchannels.

When the search mode field requests a search for GSM channels, thefollowing fields are transmitted:

TABLE 2 Field Length (bits) SF_TOTAL_EC_THRESH 5 SF_TOTAL_EC_IO_THRESH 5GSM_RXLEV_THRESH 6 GSM_T_REF_INCL 1 CDMA_TIME 0 or 6 NUM_GSM_CHAN 6The following set of fields are repeated once for each channel to besearched:

GSM_FREQ_BAND 3 ARFCN 10 BSIC_VERIF_REQ 1 BSIC 0 or 6 GSM_FRAME 0 or 19GSM_FRAME_FRACT 0 or 9The fields that are shown in Table 2 are defined as follows:

SF_(—) Serving Frequency total pilot E_(c) threshold. TOTAL_(—) If themobile station is not to use the measurement of EC_(—) total E_(c) ofthe pilots in the Serving Frequency Active Set THRESH in the GSMFrequencies periodic search procedure, the base station shall set thisfield to ‘11111’; otherwise, the base station shall set this field to┌(10 × log₁₀ (total_ec_thresh) + 120)/2┐ where total_ec_thresh isdefined by the following rule: The mobile station is not to visit anyGSM frequency if the total E_(c) of the pilots in the Serving FrequencyActive Set is greater than total_ec_thresh. SF_(—) Serving Frequencytotal pilot E_(c)/I_(o) threshold. TOTAL_(—) If the mobile station isnot to use the measurement of EC_(—) total E_(c)/I_(o) of the pilots inthe Serving Frequency Active IO_(—) Set in the GSM Frequencies periodicsearch procedure, THRESH the base station shall set this field to‘11111’; otherwise, the base station shall set this field to └−20 ×log₁₀ (total_ec_io_thresh)┘ where total_ec_io_thresh is defined by thefollowing rule: The mobile station is not to visit any GSM frequency ifthe total E_(c)/I_(o) of the pilots in the Serving Frequency Active Setis greater than total_ec_io_thresh. GSM_(—) GSM RXLEV ThresholdRXLEV_(—) The base station shall set this field to the minimum GSMTHRESH RXLEV that the mobile station is allowed to report. The GSM RXLEVis defined in Section 8.1.4 of GSM 05.08 GSM_T_(—) GSM Time ReferenceIncluded. REF_INCL This field indicates whether a GSM Time Reference isincluded in this message. If GSM Time Reference is specified in thismessage, the base station shall set this field to ‘1’; otherwise, thebase station shall set this field to ‘0’. CDMA_(—) A selected point inCDMA Time at which the MC-BS TIME knows the frame number and frameportion that is being transmitted by each of the GSM-BSSs for which theMC- BS will request the MS to search. If the GSM_T_REF_INCL is set to‘1’. the base station shall set this field to the CDMA System Time, inunits of 80 ms (modulo 64), to which the GSM_FRAME is referred. If theUSE_TIME field is set to ‘0’ the base station shall omit this field.NUM_(—) Number of GSM Channels GSM_(—) The base station shall set thisfield to the number of CHAN GSM ARFCN to search. GSM_(—) GSM Frequencyband FREQ_(—) BAND

In accordance with one embodiment of the disclosed method and apparatus,the following values are transmitted to indicate the particular GSMfrequency band:

TABLE 3 GSM FREQ BAND (binary) GSM Frequency Band 000 P-GSM 900 001E-GSM 900 010 R-GSM 900 011 DCS 1800 100 PCS 1900

ARFCN Absolute Radio Frequency Channel Number The base station shall setthis field to the Absolute Radio Frequency Channel Number to search asspecified in Section 2 of GSM 05.05. BSIC_(—) Base transceiver StationIdentity Code verification VERIF_REQ required The base station shall setthis field to ‘1’ if Base transceiver Station Identity Code verificationis required for the corresponding ARFCN; otherwise the base stationshall set it to ‘0’. BSIC Base transceiver Station Identity Code. If theBSIC_VERIF_REQ is set to ‘1’, the base station shall set this field tothe Base transceiver Station Identity Code of the GSM channel to searchas specified in Section 4.3.2 of GSM 03.03. If the BSIC_VERIF_REQ fieldis set to ‘0’ the base station shall omit this field. GSM_FRAME GSMFrame number of the frame that is being transmitted on the associatedchannel at the time identified in the associated CDMA Time field. If theGSM_T_REF_INCL is set to ‘1’, the base station shall set this field tothe GSM frame number valid at the time specified by CDMA_TIME in the GSMtarget base station, as specified in Section 3.3.2.2 of GSM 05.02. Ifthe GSM_T_REF_INCL field is set to ‘0’ the base station shall omit thisfield. GSM_(—) GSM Frame Fraction that is being transmitted on FRAME_(—)the associated channel at the time identified in the FRACT associatedCDMA Time field. If the GSM_T_REF_INCL is set to ‘1’, the base stationshall set this field to the number of 1/2{circumflex over ( )}9fractions of a GSM frame valid at the time specified by CDMA_TIME in theGSM target base station, with range 0 to (2{circumflex over ( )}9-1).The GSM frame duration is specified in Section 4.3.1 of GSM 05.02 as24/5200 s. If the GSM_T_REF_INCL field is set to ‘0’ the base stationshall omit this field.

Upon receipt of the Candidate Frequency Search Request Message 1503, theMS 1505 preferably estimates the amount of time that will be requiredfor the MS 1505 to perform the requested searches. The estimate may beperformed in any well known fashion. The estimate is transmitted to theMC-BS in a Candidate Frequency Search Response Message 1507.

In accordance with one embodiment of the disclosed method and apparatus,the MC-BS 1501 responds to the Candidate Frequency Search ResponseMessage 1507 by determining whether to perform a search, and if so, howthe search is to be performed. For example, in one embodiment, the MC-BS1501 transmits a Candidate Frequency Search Control Message indicatingthat the MS 1505 should begin performing a search at a predeterminedstart time (specified within the Control Message) and whether the searchshould be performed one time, continuously, or periodically.

The MS 1505 responds to the Control Message by performing a search basedupon the received information. The MS 1505 uses the timing informationprovided (i.e., the value provided in the CDMA Time field) to identifythe time at which an identified portion of a GSM frame was sent todetermine when to search for each GSM signal for which the MS-BS 1501has requested the MS 1505 to search.

The MS 1505 will preferably search for each GSM only at the time whenthe GSM signal is transmitting identifying information, such as theBSIC. The MS 1505 can then both make signal quality measurements andalso compare the BSIC with the BSIC associated with the channel forwhich the MS 1505 was requested to search. If there is a match, then theMS 1505 will report the quality of the signal being transmitted on thechannel for which the MS 1505 was requested to search (such as theamount of power in the signal, the signal to noise ratio, or any othermeasure of signal quality).

When the MS 1505 has determined the quality of the signal beingtransmitted on each of the channels for which the MS 1505 was requestedto search, the MS 1505 will compose a Candidate Frequency Search ReportMessage 1511. The Candidate Frequency Search Report Message 1511 is thentransmitted from the MS 1505 to the MC-BS 1501. Depending upon thecontent of the Control Message, the MS 1505 may repeatedly transmit theReport Message 1511.

If the MS-BS 1501 determines that the conditions for a handover areripe, then the MS-BS 1501 transmits messages 1513 to the GSM-BSS 1515 toprepare the GSM-BSS 1515 to accept the handover. One method used totransmit the messages to the GSM-BS 1515 are to encapsulate theinformation in a standard GSM handover message. The handover message mayinclude timing information regarding when to find the synchronizationchannel in cases in which there is substantial drift in the GSM timingwith respect to CDMA timing. Such messages are known in the art and soare not described here in detail for the sake of simplicity.

Once the GSM-BSS 1515 receives the handover preparation message 1513, anMC-MAP GSM Handover Command message 1517 is transmitted to the MS 1505in conventional GSM format. The MS 1505 and the GSM-BSS then exchangeSystem Acquisition and Access messages 1519 in essentially conventionalfashion. The MS 1505 then provides a Handover Complete Message 1521 tothe GSM-BSS 1515. The GSM-BSS 1515 and the MC-BS 1501 then exchangeHandover Complete Messages 1523.

It will be understood by those skilled in the art that if the MS 1505can quickly identify signals being transmitted from one GSM-BSS 1515,then the MS 1505 will be able to determine when to monitor for signalsbeing transmitted by other GSM-BSSs 1515 of interest. Furthermore, sincethe Candidate Frequency Search Request Message 1503 includes informationregarding each of the channels for which the MS 1505 is being requestedto search, the search for signals associated with each of these channelscan be done in a few time slots (each of which is only 0.5 millisecondsin duration). Accordingly, the presently disclosed method and apparatusallows an MS 1505 to perform a search for a handover candidate withouttaking very much time (only a few milliseconds total) from the time thatthe MS 1505 is receiving CDMA signals.

It should be noted that while the above-disclosed embodiments arereferenced to a GSM system, the presently disclosed method and apparatusapplied equally to any TDMA system in which information is transmittedduring well defined time slots. This is illustrated by the followingdescription with reference to FIG. 16 of a method, in accordance withone embodiment, of intersystem handover between two so-called “thirdgeneration” wireless communication systems, a multicarrier (MC) systemand a direct spread (DS) system.

Handover from MC System to DS System

In one embodiment message flow between an MS 1600, a source BS 1605, anda target BS 1610 during intersystem handover proceeds as illustrated inFIG. 16. The source BS 1605 is advantageously a BS of an MC system suchas, e.g., cdma2000, and the target BS1610 is advantageously a BS of a DSsystem such as, e.g., WCDMA. In the alternative, the source BS 1605 maybe a BS of an MC system that uses the mobile application part (MAP)network protocol (as opposed to an MC system such as cdma2000, whichuses the ANSI-41 network protocol), and the DS system may be a DS systemthat uses the ANSI-41 network protocol (as opposed to WCDMA, which usesthe MAP network protocol). The intersystem handover may be necessarybecause the MS 1600 has traveled out of the vicinity of an MC networkand into the vicinity of a DS network. Alternatively, the intersystemhandover may take place in an area where the two networks overlap.

The intersystem handover process begins when the source BS 1605 sends acandidate frequency search (CFS) request message 1615 to the MS 1600.The CFS request message 1615 tells the MS 1600 to search for new BSfrequencies. Timing information regarding the DS system of the target BS1610 is advantageously included with the CFS request message 1615. Suchtiming information in intersystem handover from an MC BS to a GSM BS canbe efficiently provided by absolute system because the GSM frame lengthof 4.6 ms is not an exact multiple of the MC frame length, which is 20ms. To determine DS timing for the intersystem handover from the MC BS1605 to the DS BS 1610, however, relative time (i.e., the difference intiming between two frames) may be used because the DS frame length of 10ms is an exact multiple of the MC frame length of 20 ms.

Upon receipt of the CFS request message 1615, the MS 1600 sends a CFSresponse message 1620 back to the source BS 1605 informing the source BS1605 of the time required for the MS 1600 to perform the frequencysearch. The source BS 1605 then sends a CFS control message 1625 to theMS1600 telling the MS 1600 to begin searching for BS frequencies. Whenthe MS 1600 has determined the quality of the signal being transmittedon each of the channels for which the MS 1600 was requested to search,the MS 1600 composes and sends a CFS report message 1630 to the sourceBS 1605. Depending upon the content of the CFS control message 1625, theMS1600 may repeatedly transmit the CFS report message 1630. The repeatedtransmission is shown as three CFS report messages 1630 for simplicity,but one of ordinary skill in the art would recognize that the numberneed not be restricted to three.

The CFS report messages 1630 advantageously include timing informationfrom the MS 1600 that the source BS 1605 uses to build a database oftiming variation accuracy, or “trust,” between the source MC system andthe target DS BS 1610 for all MSs involved in the intersystem handoverprocess from the MC system to the DS BS 1610. The source BS 1605advantageously develops a database maintained over time regarding theaccuracy of the timing information from the MS1600. The database canadvantageously be used by infrastructure manufactured to compute meantime difference between the source BS 1605 and the target BS 1610, andaccuracy as the variance of this difference and clock oscillationvariation. It should be pointed out that the CFS report messages 1511 inthe embodiment described with reference to FIG. 15 may also includetiming information from the MS 1505 to allow the MC-BS 1501 to determinetiming synchronization between the source MC system and the target GSMsystem.

After receiving the CFS report messages 1630, the source BS 1605determines whether the conditions for an intersystem handover aresatisfactory. If the conditions are satisfactory, the source BS 1605sends an intersystem handover command message 1635 to the MS1600. Theintersystem handover command message 1635 advantageously includes timinginformation regarding the target DS system. Such timing information inintersystem handover from an MC BS to a GSM BS can be efficientlyprovided by absolute system time because the GSM frame length of 4.6 msis not an exact multiple of the MC frame length, which is 20 ms. Todetermine DS timing for the intersystem handover from the MC BS 1605 tothe DS BS 1610, however, relative time (i.e., the difference in timingbetween two frames) may be used because the DS frame length of 10 ms isan exact multiple of the MC frame length of 20 ms.

The MS 1600 then engages in the intersystem handover process. Onceintersystem handover is complete, the MS 1600 sends an intersystemhandover complete message 1640 to the target BS 1610. It would beunderstood by those of skill that other messages (not shown), such as,e.g., a handover preparation message and a handover completion message,may be exchanged between the source BS 1605 and the target BS 1610during the intersystem handover process.

As those of skill would understand, the above-described embodimentadvantageously allows a first MS to rely to a certain degree of accuracyupon information provided to the network by a second MS regarding thetime difference between two BSs. This timing information is what thesource BS provides to the second MS in a CFS request message during anintersystem handover.

The following tables and field definitions specify intersystem handoveroperations in accordance with particular embodiments. It should be notedthat unless otherwise indicated, the tables show field name in theleft-hand column and associated bit length in the right-hand column.

MC-MAP Inter System Handover Command Message:

MSG_TAG: MAPISHCM

TABLE 4 Field Length (bits) USE_TIME 1 ACTION_TIME 0 or 6 SYS_TYPE 2

USE_TIME Use action time indicator. This field indicates whether anexplicit action time is specified in this message. If an explicit actiontime is specified in this message, the base station shall set this fieldto ‘1’; otherwise, the base station shall set this field to ‘0’.ACTION_TIME Action time. If the USE_TIME field is set to ‘1’, the basestation shall set this field to the System Time, in units of 80 ms(modulo 64), at which the handover is to take effect. If the USE_TIMEfield is set to ‘0’ the base station shall omit this field. SYS_TYPESystem Type. The base station shall set this field as specified in Table5 to denote the type of system to which the mobile station will performhandoff.

TABLE 5 System Type SYS_TYPE (binary) System 00 GSM 01 DS 10-11 Reserved

If SYS_TYPE is set to ‘00’, the base station shall include followingfields specified in Table 6:

TABLE 6 GSM_T_REF_INCL  1 CDMA_TIME  0 or 6 GSM_FN_MOD_51  0 or 6GSM_FRAME_FRACT  0 or 9 GSM_INFO_LEN 12 GSM_INFO_DATA  8 × GSM_INFO_LEN

GSM_T_REF_INCL GSM Time Reference Included. This field indicates whethera GSM Time Reference is included in this message. If GSM Time Referenceis specified in this message, the base station shall set this field to‘1’; otherwise, the base station shall set this field to ‘0’. CDMA_TIMECDMA Time. If the GSM_T_REF_INCL is set to ‘1’, the base station shallset this field to the CDMA System Time, in units of 80 ms (modulo 64),to which the GSM_FN_MOD_51 is referred. If the USE_TIME field is set to‘0’ the base station shall omit this field. GSM_FN_MOD_51 GSM Framenumber modulo 51. If the GSM_T_REF_INCL is set to ‘1’, the base stationshall set this field to the GSM frame number modulo 51 valid at the timespecified by CDMA_TIME in the GSM target base station, as specified inSection 4.3.3 of GSM 05.02. If the GSM_T_REF_INCL field is set to ‘0’the base station shall omit this field. GSM_FRAME_(—) GSM FrameFraction. FRACT If the GSM_T_REF_INCL is set to ‘1’, the base stationshall set this field to the number of 1/2⁹ fractions of a GSM framevalid at the time specified by CDMA_TIME in the GSM target base station,with range 0 to (2⁹-1). The GSM frame duration is specified in Section4.3.1 of GSM 05.02 as 24/5200 s. If the GSM_T_REF_INCL field is set to‘0’, the base station shall omit this field. GSM_INFO_LEN GSMInformation Length. The base station shall set this field to the numberof octets in the GSM_INFO_DATA fields included in this message.GSM_INFO_DATA GSM Information Data. The base station shall set thisfield as the information elements included in the Handover Command, asspecified in Section 9.1.15 of GSM 04.08 (FFS).

If SYS_TYPE is set to ‘01’, the base station shall include followingfields specified in Table 7:

TABLE 7 OPR_MODE  1 DS_T_REF_INCL  1 TIME_DIF_ACCURACY  0 or 2CHIP_INTERVALS  0 or 11 DS_INFO_LEN 12 DS_INFO_DATA  8 × DS_INFO_LEN

OPR_MODE Operating Mode. The base station shall set this field to ‘0’,if the handoff is to FDD mode of DS operation; otherwise the basestation shall set this field to ‘1’, if the handoff is to TDD mode of DSoperation. DS_T_REF_INCL DS Time Reference Included. This filedindicates whether a DS time reference is included in the message or not.If DS time reference is specified in this message, the base stationshall set this field to ‘1’; otherwise, the base station shall set thisfield to ‘0’. TIME_DIF_(—) Time Difference Accuracy. ACCURACY The fieldspecifies the unit of accuracy in terms of the number of DS chips. IfDS_T_REF_INCL is set to ‘1’, the base station shall set this fieldaccording to the corresponding value specified in Table. IfDS_T_REF_INCL is set to ‘0’, the base station shall omit this field.CHIP_INTERVALS DS Chip Intervals specifying the beginning of the P-CPICHDS frame in case of FDD DS mode or P-CCPCH DS frame in case of TDD DSmode. If DS_T_REF_INCL is set to ‘1’, the base station shall set thisfield to a value such that CHIP_INTERVALS times half the valuecorresponding to TIME_DIF_ACCURACY (specified in Table) specifies thetime difference between the end of the 20 ms MC frame and beginning ofthe next P-CPICH or P-CCPCH DS frame. DS_INFO_LEN DS Information Length.The base station shall set this field to the number of octets in theDS_INFO_DATA fields included in this message. DS_INFO_DATA DSInformation Data. The base station shall set this field as theinformation elements included in the Handover to UTRAN Command, asspecified in 3GPP TS25.331.Candidate Frequency Search Request Message: MSG_TAG: CFSRQM

TABLE 8 Field Length (bits) USE_TIME 1 ACTION_TIME 6 RESERVED_1 4CFSRM_SEQ 2 SEARCH_TYPE 2 SEARCH_PERIOD 4 SEARCH_MODE 4MODE_SPECIFIC_LEN 8 Mode-specific fields 8 × MODE_SPECIFIC_LENALIGN_TIMING 1 SEARCH_OFFSET 0 or 6

SEARCH_MODE Search mode. The base station shall set this field to theSEARCH_MODE value specified in Table 9 corresponding to the type ofsearch specified by this message.

TABLE 9 SEARCH_MODE Types SEARCH_MODE (binary) Description 0000 Searchesfor CDMA pilots on a Candidate Frequency. 0001 Searches for analogchannels. 0010 Search for GSM channels 0011 Search for FDD DS channels0100 Search for TDD DS channels 0101-1111 Reserved

If SEARCH_MODE is equal to ‘0010’, the base station shall include thefollowing fields specified in Table 10:

TABLE 10 Field Length (bits) SF_TOTAL_EC_THRESH  5 SF_TOTAL_EC_IO_THRESH 5 GSM_RXLEV_THRESH  6 BSIC_VERIF_REQ  1 N_COL_CODE  0 or 3GSM_T_REF_INCL  1 CDMA_TIME  0 or 6 GSM_T_REF_REQ  1 NUM_GSM_CHAN  5NUM_GSM_CHAN occurrences of the following record: GSM_FREQ_BAND  3 ARFCN10 CHAN_T_REF_INCL  1 GSM_FN_MOD_51  0 or 6 FRAME_FRACT_INCL  0 or 1GSM_FRAME_FRACT  0 or 9 RESERVED_6  0-7

SF_TOTAL_EC-_(—) Serving Frequency total pilot E_(c) threshold. THRESHIf the mobile station is not to use the measurement of total E_(c) ofthe pilots in the Serving Frequency Active Set in the GSM Frequenciesperiodic search procedure, the base station shall set this field to‘11111’; otherwise, the base station shall set this field to ┌(10 ×log₁₀ (total_ec_thresh) + 120)/2┐, where total_ec_thresh is defined bythe following rule: The mobile station is not to visit any GSM frequencyif the total E_(c) of the pilots in the Serving Frequency Active Set isgreater than total_ec_thresh. SF_TOTAL_EC_(—) Serving Frequency totalpilot E_(c)/I_(o) IO_THRESH threshold. If the mobile station is not touse the measurement of total E_(c)/I_(o) of the pilots in the ServingFrequency Active Set in the GSM Frequencies periodic search procedure,the base station shall set this field to ‘11111’; otherwise, the basestation shall set this field to └−20 × log₁₀ (total_ec_io_thresh)┘,where total_ec_io_thresh is defined by the following rule: The mobilestation is not to visit any GSM frequency if the total Echo of thepilots in the Serving Frequency Active Set is greater thantotal_ec_io_thresh. GSM_RXLEV_(—) GSM RXLEV Threshold. THRESH The basestation shall set this field to the minimum GSM RXLEV for which themobile station is to include a candidate frequency and to send theCandidate Frequency Search Report Message. The GSM RXLEV is defined inSection 8.1.4 of GSM 05.08. BSIC_VERIF_REQ Base transceiver StationIdentity Code verification required. The base station shall set thisfield to ‘1’ if the verification of the Network Color Code included inthe Base transceiver Station Identity Code is required for thecorresponding ARFCN (see Section A.1 of GSM 03.03); otherwise, the basestation shall set it to ‘0’. N_COL_CODE Network Color Code. If theBSIC_VERIF_REQ is set to ‘1’, the base station shall set this field tothe Network Color Code of the GSM system to search as specified inSection 4.3.2 of GSM 03.03. If the BSIC_VERIF_REQ field is set to ‘0’the base station shall omit this field. GSM_T_REF_INCL GSM TimeReference Included. This field indicates whether a GSM Time Reference isincluded in this message. If GSM Time Reference is specified in thismessage, the base station shall set this field to ‘1’; otherwise, thebase station shall set this field to ‘0’. CDMA_TIME CDMA Time. If theGSM_T_REF_INCL is set to ‘1’, the base station shall set this field tothe CDMA System Time, in units of 80 ms (modulo 64), to which theGSM_FN_MOD_51 of each GSM channel is referred. If the USE_TIME field isset to ‘0’ the base station shall omit this field. GSM_T_REF_REQ GSMTime Reference Requested. The base station shall set this field to ‘1’if a GSM Time Reference is requested to be included in the CandidateFrequency Search Report message for each reported GSM channel; otherwisethe base station shall set this field to ‘0’. NUM_GSM_CHAN Number of GSMChannels. The base station shall set this field to the number of GSMARFCN to search. The base station shall include NUM_GSM_CHAN occurrencesof the following six-field record, one for each GSM channel.GSM_FREQ_BAND GSM Frequency band. The base station shall set this fieldto the GSM Frequency Band of the GSM ARFCN to search as Table 11.

TABLE 11 GSM Frequency Band GSM_FREQ_BAND (binary) GSM Frequency Band000 P-GSM 900 001 E-GSM 900 010 R-GSM 900 011 DCS 1800 100 PCS 1900101-111 Reserved

ARFCN Absolute Radio Frequency Channel Number. The base station shallset this field to the Absolute Radio Frequency Channel Number to searchas specified in Section 2 of GSM 05.05. CHAN_T_(—) Channel TimeReference Included. REF_INCL This field indicates whether a TimeReference for this GSM channel is included in the record. If ChannelTime Reference is included in this record, the base station shall setthis field to ‘1’; otherwise, the base station shall set this field to‘0’. GSM_FN_(—) GSM Frame number modulo 51. MOD_51 If theCHAN_T_REF_INCL is set to ‘1’, the base station shall set this field tothe GSM frame number modulo 51 valid at the time specified by CDMA_TIMEin the GSM target base station, as specified in Section 4.3.3 of GSM05.02. If the CHAN_T_REF_INCL field is set to ‘0’, the base stationshall omit this field. FRAME_(—) GSM Frame Fraction included. FRACT_INCLIf the CHAN_T_REF_INCL is set to ‘1’, the base station shall set thisfield to indicate that a GSM_FRAME_FRACT for this channel is included inthe message. If the CHAN_T_REF_INCL field is set to ‘0’ the base stationshall omit this field. GSM_FRAME_(—) GSM Frame Fraction. FRACT If theFRAME_FRACT_INCL is included and set to ‘1’, the base station shall setthis field to the number of 1/2{circumflex over ( )}9 fractions of a GSMframe valid at the time specified by CDMA_TIME in the GSM target basestation, with range 0 to (2{circumflex over ( )}9-1). The GSM frameduration is specified in Section 4.3.1 of GSM 05.02 as 24/5200 s. If theFRAME_FRACT_INCL field is not included or it is set to ‘0’ the basestation shall omit this field. RESERVED_6 The mobile station shall addreserved bits as needed in order to make the length of the Mode-specificfields equal to an integer number of octets. The mobile station shallset each of these bits to ‘0’.

If SEARCH_MODE is equal to ‘0011’, the base station shall include thefollowing fields specified in Table 12:

TABLE 12 Field Length (bits) SF_TOTAL_EC_THRESH  5 SF_TOTAL_EC_IO_THRESH 5 CHIP_RATE_1  2 QSEARCH_1  6 SMIN_1  3 REP_OBS_TIME_DIF  1NUM_DS_FDD_CHAN  5 NUM_DS_FDD_CHAN occurrences of the following record:DS_FREQ_BAND_1  3 UARFCN_1 10 P_CPICH_INFO  9 P_CPICH_DL_TX_PWR  6QMIN_1  6 MAX_UL_TX_PWR_1  5 DS_FDD_T_REF_INCL  1 TIME_DIF_ACCURACY_1  0or 2 CHIP_INTERVALS_1  0 or 11 RESERVED_7  0-7

SF_TOTAL_EC_(—) Serving Frequency total pilot E_(c) threshold. THRESH Ifthe mobile station is not to use the measurement of total E_(c) of thepilots in the Serving Frequency Active Set in the GSM Frequenciesperiodic search procedure, the base station shall set this field to‘11111’; otherwise, the base station shall set this field to ┌(10 ×log₁₀ (total_ec_thresh) + 120)/2┐, where total_ec_thresh is defined bythe following rule: The mobile station is not to visit any DS frequencyif the total E_(c) of the pilots in the Serving Frequency Active Set isgreater than total_ec_thresh. SF_TOTAL_EC_IO_(—) Serving Frequency totalpilot E_(c)I_(o) THRESH threshold. If the mobile station is not to usethe measurement of total E_(c)I_(o) of the pilots in the ServingFrequency Active Set in the DS frequencies periodic search procedure,the base station shall set this field to ‘11111’; otherwise, the basestation shall set this field to └−20 × log₁₀ (total_ec_io_thresh)┘,where total_ec_io_thresh is defined by the following rule: The mobilestation is not to visit any DS frequency if the total E_(c)I_(o) of thepilots in the Serving Frequency Active Set is greater thantotal_ec_io_thresh. CHIP_RATE_1 The base station shall set this field toa number corresponding to the DS chip rate specified in Table 13.

TABLE 13 DS Chip Rate CHIP_RATE DS System Chip Rate (binary) (Mcps) 003.84 01-11 Reserved

QSEARCH_1 Quality Search Threshold. The base station shall set thisfield to the UTRA carrier received signal strength indicator (RSSI)representing a threshold the mobile shall start to measure on UMTSneighbors (see 3GPP TS25.331). If the base station doesn't haveknowledge of the minimum RSSI required for UMTS cell neighbors, it shallset all the bits of this field to ‘0’. SMIN_1 Minimum DS Cell Selectionvalue (in dB). The base station shall set this field to the minimum DScell selection value for which the mobile station is to include acandidate frequency and to send the Candidate Frequency Search ReportMessage. The cell selection value, using the measured P-CPICH Ec/Io, iscomputed by the mobile station using the fields QMIN and MAX_UL_TX_PWR(in the record below) for each DS channel as defined in 3GPP TS25.304.REP_OBS_TIME_DIF Report Observed Time Difference. The base station mayset this field to ‘1’ if it wants the mobile station to report theobserved time difference from the end of the 20 ms MC frame to thebeginning of the 10 ms P-CPICH DS frame; otherwise it shall set thisfield to ‘0’. NUM_DS_FDD_(—) Number of DS FDD Channels. CHAN The basestation shall set this field to the number of DS ARFCN to search in FDDmode. The base station shall include NUM_DS_FDD_CHAN occurrences of thefollowing ten-field record, one for each DS channel. DS_FREQ_BAND_1 Thebase station shall set this field to the DS Frequency Band specified inTable 14. The corresponding DS band is defined in 3GPP TS25.101.

TABLE 14 DS Frequency Band DS_FREQ_BAND Corresponding DS Frequency(binary) Band 000 IMT2000 001-111 Reserved

UARFCN_1 UTRA Absolute Radio Frequency Channel Number. The base stationshall set this field to UARFCN number corresponding to the frequency itwants the mobile station to search (as specified in 3GPP TS25.101).P_CPICH_INFO Primary Common Pilot Channel Information. The base stationshall set this field to the number of the Primary Scrambling Code usedin P-CPICH DS channel (see 3GPP TS25.331 and TS25.211).P_CPICH_DL_TX_PWR Primary CPICH Downlink Transmit Power (in dB). Thebase station shall set this field to the downlink transmit power of DSCPICH channel (see 3GPP TS25.331). The base station shall set all thebits of this field to ‘0’, if it does not have the information about theDS CPICH downlink transmit power. QMIN_1 Minimum required quality levelin the DS cell. The base station shall set this field to the minimumrequired quality level of the received signal Ec/Io for operating in aDS cell. QMIN is defined in 3GPP TS25.304. MAX_UL_TX_PWR_1 MaximumUplink Transmit Power (dBm). The base station shall set this field tothe maximum transmit power the mobile station may use when accessing theDS cell on DS RACH channel. This quantity is defined asUE_TXPWR_MAX_RACH in 3GPP TS25.304. If the base station doesn't haveinformation about the maximum uplink transmit power in DS neighbor cell,it shall set all the bits of this field to ‘0’. DS_FDD_T_REF_INCL DS FDDTime Reference Included. This field indicates whether a DS timereference is included in the message or not. If DS time reference isspecified in this message, the base station shall set this field to ‘1’;otherwise, the base station shall set this field to ‘0’.TIME_DIF_ACCURACY_1 Time Difference Accuracy. The field specifies theunit of accuracy in terms of the number of DS chips. IfDS_FDD_T_REF_INCL is set to ‘1’, the base station shall set this fieldaccording to the corresponding value specified in Table. IfDS_FDD_T_REF_INCL is set to ‘0’, the base station shall omit this field.

TABLE 15 Difference Time Accuracy Accuracy of the timing specification(in number of DS TIME_DIF_ACCURACY chips) 00 40 01 256 10 2560 11Reserved

CHIP_INTERVALS_1 DS Chip Intervals specifying the beginning of theP-CPICH DS frame. If DS_FDD_T_REF_INCL is set to ‘1’, the base stationshall set this field to a value such that CHIP_INTERVALS times half thevalue corresponding to TIME_DIF_ACCURACY (specified in Table 15 above)specifies the time difference between the beginning of the next P-CPICHDS frame from the end of the current MC frame. RESERVED_7 The mobilestation shall add reserved bits as needed in order to make the length ofthe Mode-specific fields equal to an integer number of octets. Themobile station shall set each of these bits to ‘0’.

If SEARCH MODE is equal to ‘0100’, the base station shall include thefollowing fields specified in Table 16:

TABLE 16 Field Length (bits) SF_TOTAL_EC_THRESH 5 SF_TOTAL_EC_IO_THRESH5 CHIP_RATE_2 2 QSEARCH_2 6 SMIN_2 3 REP_OBS_TIME_DIF 1 NUM_DS_TDD_CHAN5 NUM_DS_TDD_CHAN occurrences of the following record: DS_FREQ_BAND_2 3UARFCN_2 10 P_CCPCH_INFO 24 P_CCPCH_DL_TX_PWR 6 QMIN_2 5DS_TDD_T_REF_INCL 1 TIME_DIF_ACCURACY_2 0 or 2 CHIP_INTERVALS_2 0 or 11RESERVED_8 0-7

SF_TOTAL_EC_THRESH Serving Frequency total pilot E_(C) threshold. If themobile station is not to use the measurement of total E_(C) of thepilots in the Serving Frequency Active Set in the GSM Frequenciesperiodic search procedure, the base station shall set this field to‘11111’; otherwise, the base station shall set this field to ┌(10 ×log₁₀ (total_ec_thresh) + 120)/2┐, where total_ec_thresh is defined bythe following rule: The mobile station is not to visit any DS frequencyif the total E_(C) of the pilots in the Serving Frequency Active Set isgreater than total_ec_thresh. SF_TOTAL_EC_IO_THRESH Serving Frequencytotal pilot E_(C)/I_(O) threshold. If the mobile station is not to usethe measurement of total E_(C)/I_(O) of the pilots in the ServingFrequency Active Set in the DS frequencies periodic search procedure,the base station shall set this field to ‘11111’; otherwise, the basestation shall set this field to └−20 × log₁₀ (total_ec_io_thresh)┘,where total_ec_io_thresh is defined by the following rule: The mobilestation is not to visit any DS frequency if the total E_(C)/I_(O) of thepilots in the Serving Frequency Active Set is greater thantotal_ec_io_thresh. CHIP_RATE_2 The base station shall set this field toa number corresponding to the DS chip rate specified in Table 13 above.QSEARCH_2 Quality Search Threshold. The base station shall set thisfield to the UTRA carrier received signal strength indicator (RSSI)representing a threshold the mobile shall start to measure on UMTSneighbors (see 3GPP TS25.331). If the base station doesn't haveknowledge of the minimum RSSI required for UMTS cell neighbors, it shallset all the bits of this field to ‘0’. SMIN_2 Minimum DS cell Selectionvalue (in dB). The base station shall set this field to the minimum DScell selection value for which the mobile station is to include acandidate frequency and to send the Candidate Frequency Search ReportMessage. The cell selection value of the quantity specified in 3GPPTS25.331 (FFS) is computed by the mobile station using the cellselection procedures defined in 3GPP TS25.304 (FFS). REP_OBS_TIME_DIFReport Observed Time Difference. The base station may set this field to‘1’ if it wants the mobile station to report the observed timedifference between the beginning of the 10 ms P-CCPCH DS frame and theend of the 20 ms MC frame; otherwise it shall set this field to ‘0’.NUM_DS_TDD CHAN Number of DS TDD Channels. The base station shall setthis field to the number of DS ARFCN to search in TDD mode. The basestation shall include NUM_DS_TDD_CHAN occurrences of the followingten-field record, one for each DS channel. DS_FREQ_BAND_2 The basestation shall set this field to the DS Frequency Band specified in Table14. The corresponding DS bands are defined in 3GPP TS25.102. UARFCN_2UTRA Absolute Radio Frequency Channel Number. The base station shall setthis field to UARFCN number corresponding to the frequency it wants themobile station to search (as specified in 3GPP TS25.102). P_CCPCH_INFOPrimary Common Control Physical Channel Information. The base stationshall set this record as specified in Table 17 (see 3GPP TS25.331).

TABLE 17 Field Length (bits) TIME_SLOT 4 REP_PERIOD 3 OFFSET 6REP_LENGTH 6 BL_STTD_IND 1 RESERVED_9 4

TIME_SLOT Time slot of the TDD P-CCPCH frame. REP_PERIOD RepetitionPeriod of the P-CCPCH frame. The base station shall set this field tothe value such that repetition period is equal to 2{circumflex over( )}(REP_PERIOD). If the repetition period is not known, the basestation shall set REP_PERIOD to 0 (i.e. the repetition period of 1).OFFSET The base station shall set this field to Offset defined as SystemFrame Number (SFN) modulo the repetition period. If the offset is notknown, the base station shall set this field to its default value of 0.REP_LENGTH The base station shall set this field to the length of theallocation for each repetition. If the length is no known, the basestation shall set the field to its default value of 1. BL_STTD_IND BlockSTTD Indicator. P_CCPCH_DL_TX_PWR Primary CCPCH Downlink Transmit Power(in dB). The base station shall set this field to the downlink transmitpower of DS CPCCH channel (see 3GPP TS25.331). If the downlink transmitpower of the DS neighbor is not known, the base station shall set allthe bits of this field to ‘0’. QMIN_2 Minimum required quality level inthe DS cell. The base station shall set this field to the minimumrequired quality level of received signal quantity (specified by 3GPPTS25.304 for cell selection) for operating in a DS cell. QMIN is definedin 3GPP TS25.304. DS_TDD_T_REF_INCL DS TDD mode Time Reference Included.This field indicates whether a DS time reference is included in themessage or not. If DS time reference is specified in this message, thebase station shall set this field to ‘1’; otherwise, the base stationshall set this field to ‘0’. TIME_DIF_ACCURACY_2 Time DifferenceAccuracy. The field specifies the unit of accuracy in terms of thenumber of DS chip time. If DS_TDD_T_REF_INCL is set to ‘1’, the basestation shall set this field according to the corresponding valuespecified in Table 15 above. If DS_TDD_T_REF_INCL is set to ‘0’, thebase station shall omit this field. CHIP_INTERVALS_2 DS Chip Intervalsspecifying the beginning of the P-CPICH DS frame. If DS_TDD_T_REF_INCLis set to ‘1’, the base station shall set this field to a value suchthat CHIP_INTERVALS times half the value corresponding toTIME_DIF_ACCURACY (specified in Table 15 above) specifies the beginningof the next P-CCPCH DS frame from the end of the current MC 20 ms frame.RESERVED_8 The mobile station shall add reserved bits as needed in orderto make the length of the Mode-specific fields equal to an integernumber of octets. The mobile station shall set each of these bits to‘0’.Candidate Frequency Search Report Message: MSG_TAG: CFSRPM

TABLE 18 Field Length (bits) LAST_SRCH_MSG 1 LAST_SRCH_MSG_SEQ 2SEARCH_MODE 4 MODE_SPECIFIC_LEN 8 Mode-specific fields 8 •MODE_SPECIFIC_LEN

If SEARCH_MODE is equal to ‘0010’, the mobile station shall include thefollowing fields specified in Table 19:

TABLE 19 Field Length (bits) SF_TOTAL_RX_PWR 5 NUM_GSM_CHAN 5GSM_T_REF_INCL 1 CDMA_TIME 0 or 6 NUM_GSM_CHAN occurrences of thefollowing record: GSM_FREQ_BAND 3 ARFCN 10  BSIC 6 GSM_RXLEV 6GSM_FN_MOD_51 0 or 6 FRAME_FRACT_INCL 0 or 1 GSM_FRAME_FRACT 0 orRESERVED_4 0-7 (as needed)

SF_TOTAL_RX_PWR Indicates the total received power on the ServingFrequency. The mobile station shall set this field to min (31,┌(total_received_power + 110)/2┐), where total_received_power is themean input power received by the mobile station on the ServingFrequency, in dBm/1.23 MHz. NUM_GSM_CHAN Number of GSM Channels. Themobile station shall set this field to the number of GSM channelsincluded in this message. GSM_T_REF_INCL GSM Time Reference included.The mobile station shall set this field to ‘1’ if GSM timing informationis included for each reported GSM channel; otherwise the mobile stationshall set this field to ‘0’. CDMA_TIME CDMA Time. If the GSM_T_REF_INCLis set to ‘1’, the mobile station shall set this field to the CDMASystem Time, in units of 80 ms (modulo 64), to which the GSM_FN_MOD_51of each GSM channel is referred. If the USE_TIME field is set to ‘0’ themobile station shall omit this field.The mobile station shall include NUM_GSM_CHAN occurrences of thefollowing four-field record, one for each GSM channel:

GSM_FREQ_BAND GSM Frequency band. The mobile station shall set thisfield to the GSM Frequency Band of the reported ARFCN. ARFCN AbsoluteRadio Frequency Channel Number. The mobile station shall set this fieldto the Absolute Radio Frequency Channel Number of the reported GSMchannel as specified in Section 2 of GSM 05.05. BSIC Base transceiverStation Identity Code. The mobile station shall set this field to theBase transceiver Station Identity Code of the reported GSM channel asspecified in Section 4.3.2 of GSM 03.03. GSM_RXLEV GSM RXLEV. The mobilestation shall set this field to the GSM RXLEV of the reported GSMchannel as specified in Section 8.1.4 of GSM 05.08. RESERVED_4 Reserved.

The mobile station shall add reserved bits as needed in order to makethe length of the Mode-specific fields equal to an integer number ofoctets. The mobile station shall set each of these bits to ‘0’.

GSM_FN_MOD_51 GSM Frame number modulo 51. If the GSM_T_REF_INCL is setto ‘1’, the mobile station shall set this field to the GSM frame numbermodulo 51 valid at the time specified by CDMA_TIME in the GSM targetbase station, as specified in Section 4.3.3 of GSM 05.02. If theGSM_T_REF_INCL field is set to ‘0’, the mobile station shall omit thisfield. FRAME_FRACT_INCL GSM Frame Fraction included. If theGSM_T_REF_INCL is set to ‘1’, the mobile station shall set this field toindicate that a GSM_FRAME_FRACT for this channel is included in themessage. If the GSM_T_REF_INCL field is set to ‘0’ the mobile stationshall omit this field. GSM_FRAME_FRACT GSM Frame Fraction. If theFRAME_FRACT_INCL is included and set to ‘1’, the mobile station shallset this field to the number of 1/2{circumflex over ( )}9 fractions of aGSM frame valid at the time specified by CDMA_TIME in the GSM targetbase station, with range 0 to (2{circumflex over ( )}9-1). The GSM frameduration is specified in Section 4.3.1 of GSM 05.02 as 24/5200 s. If theFRAME_FRACT_INCL field is not included or it is set to ‘0’ the mobilestation shall omit this field.

If SEARCH MODE is equal to ‘0011’, the mobile station shall include thefollowing fields specified in Table 20:

TABLE 20 Field Length (bits) SF_TOTAL_RX_PWR 5 NUM_DS_FDD_CHAN 5NUM_DS_FDD_CHAN occurrences of the following record: DS_FDD_FREQ_BAND 3UARFCN_1 10 P_CPICH_INFO 9 P_CPICH_ECIO 6 P_CPICH_RSCP_INCL 1P_LOSS_INCL 1 CELL_ID_INCL 1 TIME_DIF_INCL 1 P_CPICH_RSCP 0 or 8PATHLOSS 0 or 7 CELL_ID 0 or 30 MC_DS_TIME_DIF 0 or 11 RESERVED_5 0-7(as needed)

SF_TOTAL_RX_PWR Indicates the total received power on the ServingFrequency. The mobile station shall set this field to min (31,┌(total_received_power + 110)/2┐), where total_received_power is themean input power received by the mobile station on the ServingFrequency, in dBm/1.23 MHz. NUM_DS_FDD_CHAN Number of DS FDD Channels.The mobile station shall set this field to the number of DS FDD channelsincluded in this message.The mobile station shall include NUM_DS_FDD_CHAN occurrences of thefollowing record, one for each reported DS FDD channel.

DS_FDD_FREQ_BAND DS FDD Frequency band. The mobile station shall setthis field to the DS Frequency Band of the reported UARFCN as specifiedTable 14 above. UARFCN_2 UTRA Absolute Radio Frequency Channel Number.The mobile station shall set this field to the Absolute Radio FrequencyChannel Number of the reported DS FDD channel as specified in 3GPPTS25.331. P_CPICH_INFO Primary Common Pilot Channel Information. Themobile station shall set this field to the Primary Scrambling Code ofthe reported P- CPICH DS channel (see 3GPP TS25.331). P_CPICH_EC_IOP-CPICH Ec/Io. The mobile station shall set this field to received Ec/Ioof the P-CPICH DS channel (dB) (see TS25.331). P_CPICH_RSCP_INCL P-CPICHReceived Signal Code Power Included. The mobile station shall set thisflag to ‘1’ if it has the P-CPICH DS channel received code powermeasurement; otherwise it shall this field to ‘0’. P_LOSS_INCL PathlossIncluded. The mobile station shall set this flag to ‘1’ if it has thePathloss measurement; otherwise it shall this field to ‘0’. CELL_ID_INCLCELL ID Included. The mobile station shall set this flag to ‘1’ if it isable to read the CELL ID during the search; otherwise it shall thisfield to ‘0’, TIME_DIF_INCL Time Difference Included. The mobile stationshall set this field to ‘1’. if time difference between MC frame and DSCPICH frame is included. P_CPICH_RSCP P-CPICH Received Signal CodePower. The mobile station shall include this field only whenP_CPICH_RSCP_INCL is set to ‘1’. If included, it shall set this field tothe value = −RSCP-40 (dBm) (see TS25.331). P_CPICH_SIR P-CPICH Signal toInterference Ratio. The mobile station shall include this field onlywhen P_CPICH_SIR_INCL is set to ‘1’. If included, it shall set thisfield to SIR + 10 (dB) (see 3GPP TS25.331). PATHLOSS Downlink Path Loss.The mobile station shall include this field only when P_LOSS_INCL is setto ‘1’. If included, it shall set this field to the value Pathloss −46(dB) (see 3GPP TS25.331). CELL_ID Cell Identification. The mobilestation shall include this field only when CELL_ID_INCL is set to ‘1’.If included, it shall set this field to the 30 bit Cell Identification(see 3GPP TS25.331). MC_DS_TIME_DIF MC and DS CPICH frame timedifference. The mobile station shall include this field only whenTIME_DIF_INCL is set to ‘1’. If included, the mobile station shall setthis field to the time difference between the end of the 20 ms MC frameand the beginning of 10 ms P-CPICH DS frame measured in units of 20 DSchips with an accuracy of 40 DS chips. RESERVED_5 The mobile stationshall add reserved bits as needed in order to make the length of theMode-specific fields equal to an integer number of octets. The mobilestation shall set each of these bits to ‘0’.

If SEARCH_MODE is equal to ‘0100’, the mobile station shall include thefollowing fields specified in Table 21:

TABLE 21 Field Length (bits) SF_TOTAL_RX_PWR 5 NUM_DS_TDD_CHAN 5NUM_DS_TDD_CHAN occurrences of the following record: DS_TDD_FREQ_BAND 3UARFCN_2 10 P_CCPCH_TIME_SLOT 6 P_CCPCH_QUALITY 6 P_CCPCH_RSCP_INCL 1TIME_DIF_INCL 1 P_CCPCH_RSCP 0 or 8 MC_DS_TIME_DIF 0 or 11 RESERVED_60-7 (as needed)

SF_TOTAL_RX_PWR Indicates the total received power on the ServingFrequency. The mobile station shall set this field to min (31,┌(total_received_power + 110)/2┐), where total_received_power is themean input power received by the mobile station on the ServingFrequency, in dBm/1.23 MHz. NUM_DS_TDD_CHAN Number of DS TDD Channels.The mobile station shall set this field to the number of DS TDD channelsincluded in this message.The mobile station shall include NUM_DS_TDD_CHAN occurrences of thefollowing record, one for each DS FDD channel.

DS_TDD_FREQ_BAND DS TDD Frequency band. The mobile station shall setthis field to the DS Frequency Band of the reported UARFCN as specifiedin Table 14. UARFCN_2 UTRA Absolute Radio Frequency Channel Number. Themobile station shall set this field to the Absolute Radio FrequencyChannel Number of the reported DS TDD channel as specified in 3GPPTS25.331. P_CCPCH_TIME_SLOT Primary CCPCH DS TDD channel time slot. Themobile station shall set this field to the time slot number specified in3GPP TS25.331 and TS25.211. P_CCPCH_QUALITY P-CCPCH Quality level. Themobile station shall set this field to the quality level of the quantitythat the mobile station uses to compute cell selection value as definedin 3GPP TS25.331 (FFS). P_CCPCH_RSCP_INCL P-CCPCH Received Signal CodePower Included. The mobile station shall set this flag to ‘1’ if it hasthe P-CCPCH DS channel received code power measurement; otherwise itshall this field to ‘0’. TIME_DIF_INCL Time Difference Included. Themobile station shall set this field to ‘1’, if time difference betweenMC frame and DS TDD P-CCPCH frame is included. P_CCPCH_RSCP P-CCPCHReceived Signal Code Power. The mobile station shall include this fieldonly when P_CCPCH_RSCP_INCL is set to ‘1’. If included, it shall setthis field to the value = −RSCP-40 (dBm) (see 3GPP TS25.331).MC_DS_TIME_DIF MC and DS CCPCH frame time difference. The mobile stationshall include this field only when TIME_DIF_INCL is set to ‘1’. Ifincluded, the mobile station shall set this field to the time differencebetween the end of the 20 ms MC frame and the beginning of 10 ms P-CCPCHDS TDD frame measured in units of 20 DS chips with an accuracy of 40 DSchips. RESERVED_6 The mobile station shall add reserved bits as neededin order to make the length of the Mode-specific fields equal to aninteger number of octets. The mobile station shall set each of thesebits to ‘0’.

Thus, a novel and improved method and apparatus for conductingintersystem handover has been described. Those of skill in the art wouldunderstand that the various illustrative logical blocks, modules,circuits, and algorithm steps described in connection with theembodiments disclosed herein may be implemented as electronic hardware,computer software, or combinations of both. The various illustrativecomponents, blocks, modules, circuits, and steps have been describedgenerally in terms of their functionality. Whether the functionality isimplemented as hardware or software depends upon the particularapplication and design constraints imposed on the overall system.Skilled artisans recognize the interchangeability of hardware andsoftware under these circumstances, and how best to implement thedescribed functionality for each particular application. As examples,the various illustrative logical blocks, modules, circuits, andalgorithm steps described in connection with the embodiments disclosedherein may be implemented or performed with a digital signal processor(DSP), an application specific integrated circuit (ASIC), a fieldprogrammable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components such as,e.g., registers and FIFO, a processor executing a set of firmwareinstructions, any conventional programmable software module and aprocessor, or any combination thereof. The processor may advantageouslybe a microprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine.The software module could reside in RAM memory, flash memory, ROMmemory, EPROM memory, EEPROM memory, registers, hard disk, a removabledisk, a CD-ROM, or any other form of storage medium known in the art.Those of skill would further appreciate that the data, instructions,commands, information, signals, bits, symbols, and chips that may bereferenced throughout the above description are advantageouslyrepresented by voltages, currents, electromagnetic waves, magneticfields or particles, optical fields or particles, or any combinationthereof.

Although preferred embodiments are described hereinabove with referenceto particular hybrid GSM/CDMA systems and hybrid DS/MC systems, it willbe appreciated that the principles of the present invention maysimilarly be applied to effect mobile-assisted handovers in other hybridcommunication systems, as well. Moreover, although the preferredembodiments make reference to specific TDMA- and CDMA-basedcommunications standards, those skilled in the art will appreciate thatthe methods and principles described above may also be used inconjunction with other methods of data encoding and signal modulation.The scope of the present invention encompasses not only the completesystems and communications processes described hereinabove, but alsovarious innovative elements of these systems and processes, as well ascombinations and sub-combinations thereof.

It will thus be appreciated that the preferred embodiments describedabove are cited by way of example, and the full scope of the inventionis limited only by the claims.

1. A wireless communications system, comprising: a base station of afirst type which transmits a first signal according to a first airinterface; a base station of a second type which transmits a secondsignal according to a second air interface; and a mobile station, whichreceives the second signal over the second air interface from the basestation of the second type while maintaining a communication link overthe first air interface with the base station of the first type, andwhich transmits data to the base station of the first type responsive tothe second signal so that the mobile station is handed over from thefirst to the second base station responsive to the transmitted data,wherein one of the first and second air interfaces comprises a TDMAinterface, and the other of the interfaces comprises a CDMA interface,the mobile station uses a single radio resource management protocollayer to manage both the first and second air interfaces.
 2. A mobilestation operable in a wireless communications system, comprising: meansfor receiving a first signal according to a first air interface from afirst base station; means for receiving a second signal over a secondair interface from a second base station while maintaining acommunications link of the first air interface with the first basestation wherein one of the first and second airinterfaces comprises aTDMA interface, and the other of the interfaces comprises a CDMAinterface; means for transmitting data to the first basestationresponsive to the second signal; means for handing over themobile station from the first base station to the second base stationresponsive to the transmitted data; and means for using a single radioresource management protocol layer to manage both the first and secondair interfaces.
 3. A mobile station operable in a wirelesscommunications system, comprising: means for receiving a first signalaccording to a first air interface from a first base station; means forreceiving a second signal over a second air interface from a second basestation while maintaining a communications link of the first airinterface with the first base station, wherein one of the first andsecond air interfaces comprise a TDMA interface, and the other of theinterfaces comprises a CDMA interface, and wherein the first basestation gates the mobile station to interrupt the communications link soas to receive and decode a TDMA signal; means for transmitting data tothe first base station responsive to the second signal; means forhanding over the mobile station from the first base station to thesecond base station responsive to the transmitted data; and means forusing a single radio resource management protocol layer to manage boththe first and second air interfaces.
 4. The mobile station according toclaim 3 further comprising means for interrupting the communicationslink for the duration of an IS-95 frame.
 5. The mobile station accordingto claim 3 further comprising means for processing the second signal todecode GSM frequency correction and synchronization channels of thesecond signal.
 6. A mobile station operable in a wireless communicationssystem, comprising: means for receiving a first signal according to afirst air interface from a first base station; means for receiving asecond signal over a second air interface from a second base stationwhile maintaining a communications link of the first air interface withthe first base station, wherein one of the first and second airinterfaces comprise a TDMA interface, and the other of the interfacescomprises a CDMA interface; means for receiving a control signal fromthe first base station to interrupt the communications link so as toreceive a CDMA signal; means for transmitting data to the first basestation responsive to the second signal; means for handing the mobilestation over from the first base station to the second base stationresponsive to the transmitted data; and means for using a single radioresource management protocol layer to manage both the first and secondair interfaces.
 7. The mobile station according to claim 6 furthercomprising means for decoding a synchronization channel of the CDMAsignal so as to derive the time of day.
 8. The mobile station accordingto claim 6 further comprising means for decoding the CDMA signal toidentify a CDMA pilot beam.
 9. The mobile station according to claim 6further comprising means for decoding the CDMA signal during a firstTDMA time slot and means for processing the signal during a subsequentTDMA time slot while communicating with the first base station over theTDMA interface.
 10. A mobile station operable in a wirelesscommunications system, comprising: circuitry configured to: receive afirst signal according to a first air interface from a first basestation; receive a second signal over a second air interface from asecond base station while maintaining a communications link of the firstair interface with the first base station, wherein one of the first andsecond air interfaces comprises a TDMA interface, and the other of theinterfaces comprises a CDMA interface; transmit data to the first basestation responsive to the second signal; hand over the mobile stationfrom the first base station to the second base station responsive to thetransmitted data; and use a single radio resource management protocollayer to manage both the first and second air interfaces.
 11. A mobilestation operable in a wireless communications system, comprising:circuitry configured to: receive a first signal according to a first airinterface from a first base station; receive a second signal over asecond air interface from a second base station while maintaining acommunications link of the first air interface with the first basestation, wherein one of the first and second air interfaces comprise aTDMA interface, and the other of the interfaces comprises a CDMAinterface, and wherein the first base station gates the mobile stationto interrupt the communications link so as to receive and decode a TDMAsignal; transmit data to the first base station responsive to the secondsignal; hand over the mobile station from the first base station to thesecond base station responsive to the transmitted data; and use a singleradio resource management protocol layer to manage both the first andsecond air interfaces.
 12. The mobile station according to claim 11wherein the circuitry is further configured to interrupt thecommunications link for the duration of an IS-95 frame.
 13. The mobilestation according to claim 11 wherein the circuitry is furtherconfigured to process the second signal to decode GSM frequencycorrection and synchronization channels of the second signal.
 14. Amobile station operable in a wireless communications system, comprising:circuitry configured to: receive a first signal according to a first airinterface from a first base station; receive a second signal over asecond air interface from a second base station while maintaining acommunications link of the first air interface with the first basestation, wherein one of the first and second air interfaces comprise aTDMA interface, and the other of the interfaces comprises a CDMAinterface; receive a control signal from the first base station tointerrupt the communications link so as to receive a CDMA signal;transmit data to the first base station responsive to the second signal;hand over the mobile station from the first base station to the secondbase station responsive to the transmitted data; and use a single radioresource management protocol layer to manage both the first and secondair interfaces.
 15. The mobile station according to claim 14 wherein thecircuitry is further configured to decode a synchronization channel ofthe CDMA signal so as to derive the time of day.
 16. The mobile stationaccording to claim 14 wherein the circuitry is further configured toprocess the CDMA signal to identify a CDMA pilot beam.
 17. The mobilestation according to claim 14 wherein the circuitry is furtherconfigured to receive the CDMA signal during a first TDMA time slot andprocess the signal during a subsequent TDMA time slot whilecommunication with the first base station over the TDMA interface.